App crashes with "Don't keep activities" setting

This sample assumes that it is run on a device with sufficient memory to make sure that the activity launching the authentication flow is still alive to receive the result via AuthenticationCallback. This assumption might not be true on low-memory devices.
Android's "Don't keep activities" allows to simulate a scenario where the Android system has killed the activity.
When running the app under this condition, it crashes:

```
java.lang.NullPointerException
at com.azuresamples.msalandroidkotlinapp.SingleAccountModeFragment.callGraphAPI(SingleAccountModeFragment.kt:307)
at com.azuresamples.msalandroidkotlinapp.SingleAccountModeFragment.access$callGraphAPI(SingleAccountModeFragment.kt:58)
at com.azuresamples.msalandroidkotlinapp.SingleAccountModeFragment$authInteractiveCallback$1.onSuccess(SingleAccountModeFragment.kt:279)
at com.microsoft.identity.client.PublicClientApplication.postAuthResult(PublicClientApplication.java:2351)
at com.microsoft.identity.client.SingleAccountPublicClientApplication$3.onTaskCompleted(SingleAccountPublicClientApplication.java:443)
at com.microsoft.identity.client.SingleAccountPublicClientApplication$3.onTaskCompleted(SingleAccountPublicClientApplication.java:434)
at com.microsoft.identity.common.java.controllers.CommandDispatcher.commandCallbackOnTaskCompleted(CommandDispatcher.java:650)
at com.microsoft.identity.common.java.controllers.CommandDispatcher.access$1000(CommandDispatcher.java:99)
at com.microsoft.identity.common.java.controllers.CommandDispatcher$4.run(CommandDispatcher.java:626)
at android.os.Handler.handleCallback(Handler.java:942)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loopOnce(Looper.java:201)
at android.os.Looper.loop(Looper.java:288)
at android.app.ActivityThread.main(ActivityThread.java:7872)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:936)
```

Start in SingleAccountModeFragment.kt around callGraphAPI at line 307 and authInteractiveCallback at line 279. Reproduce the crash with Android's "Don't keep activities" setting, then verify the authentication result no longer causes a NullPointerException when the activity has been recreated.

Written by the indexing model from the issue text.