Fix crash for UiBlockerController

1. Don't create UiBlockerController if there is no related controller
2. Don't update visibility if UiBlockerController is null
3. Use findPreference() from DashboardFragment, which already has null
check.

Change-Id: Iee24c64317fb9d5a1cf2076d25728af485d390c5
Fixes: 122807414
Fixes: 122805831
Test: RunSettingsRoboTests
3 files changed