SystemUI: Set brightness slider view restriction from the right thread

The brightness slider view can only be safely touched from the main
handler, not the background handler.

This fixes the following crash:
E AndroidRuntime: FATAL EXCEPTION: SysUiBg
E AndroidRuntime: Process: com.android.systemui, PID: 4217
E AndroidRuntime: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
E AndroidRuntime: 	at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:8804)
E AndroidRuntime: 	at android.view.ViewRootImpl.focusableViewAvailable(ViewRootImpl.java:4612)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.ViewGroup.focusableViewAvailable(ViewGroup.java:962)
E AndroidRuntime: 	at android.view.View.setFlags(View.java:16122)
E AndroidRuntime: 	at android.view.View.setEnabled(View.java:11744)
E AndroidRuntime: 	at com.android.systemui.settings.ToggleSliderView.setEnforcedAdmin(ToggleSliderView.java:101)
E AndroidRuntime: 	at com.android.systemui.settings.BrightnessController$10.run(BrightnessController.java:432)
E AndroidRuntime: 	at android.os.Handler.handleCallback(Handler.java:938)
E AndroidRuntime: 	at android.os.Handler.dispatchMessage(Handler.java:99)
E AndroidRuntime: 	at android.os.Looper.loop(Looper.java:223)
E AndroidRuntime: 	at android.os.HandlerThread.run(HandlerThread.java:67)

Change-Id: Iac48d0b652f148411c5d7a48b19d7ba0c9c24fca