about “Required assets to build and debug are missing from 'HelloCS'. Add them?” dialog

## Issue Description ##
## Steps to Reproduce ##
1. open a new folder
2. create a new console application
3. use view->command palette->Omnisharp: select project to select the new project
4. “Required assets to build and debug are missing from 'HelloCS'. Add them?” dialog is displayed on the right bottom corner
5. click "Don't Ask Again"
6. go to step 3
7. “Required assets to build and debug are missing from 'HelloCS'. Add them?” dialog will be displayed again...
8. Also in extension settings, there's no check mark for the below setting, after I click "Don't Ask Again"
Csharp: Supress Build Assets Notification
Suppress the notification window to add missing assets to build or debug the application.

## Expected Behavior ##
“Required assets to build and debug are missing from 'HelloCS'. Add them?” dialog **will not be** displayed again

## Actual Behavior ##
“Required assets to build and debug are missing from 'HelloCS'. Add them?” dialog **is** displayed again

## Logs ##

### OmniSharp log ###
Starting OmniSharp server at 2020/3/31 下午9:51:38
Target: c:\Users\iouzz\Documents\Code\test\test3

OmniSharp server started.
Path: C:\Users\iouzz\.vscode\extensions\ms-dotnettools.csharp-1.21.15\.omnisharp\1.34.14\OmniSharp.exe
PID: 12432

[info]: OmniSharp.Stdio.Host
Starting OmniSharp on Windows 6.2.9200.0 (x64)
[info]: OmniSharp.Services.DotNetCliService
DotNetPath set to dotnet
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
Located 2 MSBuild instance(s)
1: Visual Studio Community 2019 16.5.29911.84 - "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin"
2: StandAlone 16.4 - "C:\Users\iouzz\.vscode\extensions\ms-dotnettools.csharp-1.21.15\.omnisharp\1.34.14\.msbuild\Current\Bin"
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
Registered MSBuild instance: Visual Studio Community 2019 16.5.29911.84 - "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin"
[info]: OmniSharp.Cake.CakeProjectSystem
Detecting Cake files in 'c:\Users\iouzz\Documents\Code\test\test3'.
[info]: OmniSharp.Cake.CakeProjectSystem
Could not find any Cake files
[info]: OmniSharp.MSBuild.ProjectSystem
No solution files found in 'c:\Users\iouzz\Documents\Code\test\test3'
[info]: OmniSharp.MSBuild.ProjectManager
Queue project update for 'c:\Users\iouzz\Documents\Code\test\test3\test3.csproj'
[info]: OmniSharp.Script.ScriptProjectSystem
Detecting CSX files in 'c:\Users\iouzz\Documents\Code\test\test3'.
[info]: OmniSharp.Script.ScriptProjectSystem
Could not find any CSX files
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.CSharpFormattingWorkspaceOptionsProvider, Order: 0
[info]: OmniSharp.MSBuild.ProjectManager
Loading project: c:\Users\iouzz\Documents\Code\test\test3\test3.csproj
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.RenameWorkspaceOptionsProvider, Order: 100
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.ImplementTypeWorkspaceOptionsProvider, Order: 110
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.EditorConfigWorkspaceOptionsProvider, Order: 200
[info]: OmniSharp.WorkspaceInitializer
Configuration finished.
[info]: OmniSharp.Stdio.Host
Omnisharp server running using Stdio at location 'c:\Users\iouzz\Documents\Code\test\test3' on host 9976.
[info]: OmniSharp.MSBuild.ProjectManager
Successfully loaded project file 'c:\Users\iouzz\Documents\Code\test\test3\test3.csproj'.
[info]: OmniSharp.MSBuild.ProjectManager
Adding project 'c:\Users\iouzz\Documents\Code\test\test3\test3.csproj'
[info]: OmniSharp.MSBuild.ProjectManager
Update project: test3

### C# log ###
Post the output from Output-->C# here

## Environment information ##

**VSCode version**: 1.43.2
**C# Extension**: 1.21.15

Dotnet Information
.NET Core SDK����ӳ�κ� global.json��:
Version: 3.1.201
Commit: b1768b4ae7

����ʱ����:
OS Name: Windows
OS Version: 10.0.18363
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.201\

Host (useful for support):
Version: 3.1.3
Commit: 4a9f85e9f8

.NET Core SDKs installed:
3.1.200 [C:\Program Files\dotnet\sdk]
3.1.201 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|csharp|ms-dotnettools|1.21.15|;