Fix NPE when config is updated before ViewRootImpl is set

If activity received a configuration change before it had its
ViewRootImpl initialize, then there was a NPE when trying to
notify the ViewRootImpl instance about latest changes.

Bug: 36396248
Test: android.server.cts.ActivityManagerAppConfigurationTests
Test: #testLaunchWithUiModeChange
Test: Launch AndroidAuto app
Change-Id: I71e0f316f885a30dadce388bbd3d9f21f419076b