Add UiBlocker for DashboardFragment

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

src/com/android/settings/slices/SlicePreferenceController.java

@@ -51,8 +51,7 @@ public class SlicePreferenceController extends BasePreferenceController implemen
     public void displayPreference(PreferenceScreen screen) {
         super.displayPreference(screen);
 
-        mSlicePreference = (SlicePreference) screen.findPreference(
-                getPreferenceKey());
+        mSlicePreference = screen.findPreference(getPreferenceKey());
     }
 
     @Override