Relax Flutter SDK validation in the settings field

## Steps to Reproduce

When I open the Flutter framework to work on it, the "No Flutter SDK Configured" banner keeps reappearing.

1) Open the Flutter framework in the IDE
2) see the "No Flutter SDK Configured" banner at the top of a Dart file.
3) Click on "Open Flutter Settings"
4) Select your flutter directory.
5) Wait until the version information populates. Mine says:
```
Flutter 1.18.0-9.0.pre.33 • channel ignore_focus • git@github.com:gspencergoog/flutter.git
Framework • revision dc6c453262 (6 hours ago) • 2020-04-27 11:40:08 -0700
Engine • revision 4bcfae82c7
Tools • Dart 2.9.0 (build 2.9.0-3.0.dev a69cb6d700)
```
6) Click "Apply"
7) Notice that the selected flutter SDK field blanks out again
8) Select the flutter directory again
5) Wait until the version information populates again.
9) Click "OK".
10) Notice that:
a) The banner didn't go away
b) When I reopen the settings page, the SDK path is blank again.
c) Even if I click "dismiss", closing the tab and reopening that file brings back the banner.


## Version info

I also do have IntelliJ installed, just not in a standard location that flutter doctor can find it in.
I'm running IntelliJ CE 2020.1, with the 46.0.3-dev.3 version of the Flutter plugin.

```
[✓] Flutter (Channel ignore_focus, v1.18.0-9.0.pre.33, on Mac OS X 10.15.4
19E287, locale en-US)
• Flutter version 1.18.0-9.0.pre.33 at /Users/gspencer/code/flutter
• Framework revision dc6c453262 (6 hours ago), 2020-04-27 11:40:08 -0700
• Engine revision 4bcfae82c7
• Dart version 2.9.0 (build 2.9.0-3.0.dev a69cb6d700)

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
• Android SDK at /Users/gspencer/Library/Android/sdk
• Platform android-29, build-tools 29.0.2
• ANDROID_HOME = /Users/gspencer/Library/Android/sdk
• Java binary at: /Applications/Android
Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build
1.8.0_202-release-1483-b49-5587405)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.3)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 11.3, Build version 11C29
• CocoaPods version 1.9.1

[✓] Chrome - develop for the web
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 3.5)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin version 38.2.3
• Dart plugin version 191.8423
• Java version OpenJDK Runtime Environment (build
1.8.0_202-release-1483-b49-5587405)

[!] VS Code (version 1.43.2)
• VS Code at /Applications/Visual Studio Code.app/Contents
✗ Flutter extension not installed; install from
https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter

[✓] Connected device (3 available)
• macOS • macOS • darwin-x64 • Mac OS X 10.15.4 19E287
• Web Server • web-server • web-javascript • Flutter Tools
• Chrome • chrome • web-javascript • Google Chrome 81.0.4044.122
```