Fix launcher failing tests.

- Check for null before accessing top-display-focus stack.
- Create home stack if it doesn't exist.

Fixes: 144030618
Test: atest NexusLauncherOutOfProcTests
Test:
1) Clear Recents
2) adb shell am force-stop com.google.android.apps.nexuslauncher
3) Press home button
Change-Id: Ibb5e5dfafcabdc4af35d6544cdc1d5bf7e2c8eee