Avoid the scenario where Contexts list filled with null value in ShowAnrDialogs()

Problem: Framework Reboot i.e., NullPointer Exception in android.ui Thread
E AndroidRuntime: FATAL EXCEPTION IN SYSTEM PROCESS: android.ui
E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method
'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference

E AndroidRuntime: **at android.app.Dialog.<init>(Dialog.java:199)
E AndroidRuntime: **at android.app.AlertDialog.<init>(AlertDialog.java:204)
E AndroidRuntime: **at android.app.AlertDialog.<init>(AlertDialog.java:200)
E AndroidRuntime: **at com.android.server.am.BaseErrorDialog.<init>(BaseErrorDialog.java:39)
E AndroidRuntime: **at com.android.server.am.AppNotRespondingDialog.<init>(AppNotRespondingDialog.java:53)
E AndroidRuntime: **at com.android.server.am.ErrorDialogController.showAnrDialogs(ErrorDialogController.java:183)
E AndroidRuntime: **at com.android.server.am.AppErrors.handleShowAnrUi(AppErrors.java:1077)

Analysis:
As Context Object is passed from showAnrDialogs method and will be retrieved from getDisplayContextWithErrorDialogs
And list of context objects will be filled by querying from RootWindowContainer.getDisplayUiContext and this method is
Annotated with Nullable. So, the possibility could be null value filled in the context object List.

Bug: 196189977
Test: Stability Test

Change-Id: Ib944fc5ffc8bcca999c75f99d462d441e555fcda