Microsoft Graph Picture Cache

## I'm submitting a...
- Bug report (I searched for similar issues and did not find one)

## Current behavior
If you login with a Work Account (which supports the user picture endpoint), logoff and login with an MSA (which does not supports the user picture endpoint), the old Work Account user picture is shown.

## Expected behavior
The sample app should not show another user's picture, if it can't find mine.

## Minimal reproduction of the problem with instructions
1) Open the sample app
2) Open the Microsoft Graph Sample
3) Login with an account which will return a picture profile (a Work or School account)
4) Logoff
5) Login with an account which will not return a picture profile (an MSA account)

## Environment

```
Nuget Package(s): Microsoft.Toolkit.Uwp.Services

Package Version(s): Latest from source (3.0 preview)

Windows 10 Build Number:
- [ ] Anniversary Update (14393)
- [ ] Creators Update (15063)
- [ ] Fall Creators Update (16299)
- [X] Insider Build (build number: 17672)

App min and target version:
- [Min] Anniversary Update (14393)
- [ ] Creators Update (15063)
- [Target] Fall Creators Update (16299)
- [ ] Insider Build (xxxxx)

Device form factor:
- [X] Desktop
- [ ] Mobile
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio
- [X] 2017 (version: 15.7.1)
- [ ] 2017 Preview (version: )

```