AppErrors: Add test for AppErrorDialog.onCreate

Also refactor to make sure we always remove the TIMEOUT
message and clear the mProc.mCrashDialog field, even when
dismissing without the Handler.

Bug: 37351370
Test: runtest -c com.android.server.am.AppErrorDialogTest frameworks-services
Change-Id: If9fb348e8ed83c6c1c0f48fa7fd27ffd33b530f2