If DashboardFragment detects blocker controllers, it won't display
until:
1. All blocking controllers finish bg works.
2. Timeout
This CL adds UiBlockerController to control this behavior and
BlockingSlicePreferenceController as an example.
Bug: 120803703
Test: atest SettingsUnitTests
Change-Id: I66fc194776d46ee49b3ec7685f3167834e673ba2
