Do less on the main thread

With a settings ContentObserver. However, unlike the
last time only the binder-related work is happening on a
background thread, and everything else is on the main thread.

Test: android.platform.test.scenario.sysui.bubble.CreateBubbleFromNotificationTest#testCreateBubbleFromNotification
Test: ExpandableNotificationRowControllerTest
Test: NotificationSettingsControllerTest
Test: NotificationContentViewTest
Fixes: 271435682
Change-Id: I2d1f5c52107409a6c1081fb3000035046202cad3