Improve auto-brightness handling

- Add a setting to control the auto-brightness algorithm responsitivity
- Make the algorithm react much faster when the ambient brightness
  increases quickly, e.g. when receiving a call in bright sunlight.

Change-Id: Ic608901c476609dde69cae982228c09567140a83

Conflicts:
	services/java/com/android/server/power/DisplayPowerRequest.java
	services/java/com/android/server/power/PowerManagerService.java
Reduce observers in SystemUI: StatusBarIconView

Currently, for each status bar icon view there is a observer that
listens for changes of STATUS_BAR_NOTIF_COUNT. With one
notification usually having one or more status bar icons
you may end up easily with 10+ more observers for ~5 notifications.

With this commit the internal observer class becomes a singleton.
This means there is now only one observer for all status bar icons
and additional binder calls on new notification are avoided.

See "dumpsys content"

Change-Id: I7ca6481fe43e87c34b19d7b71e531cce93cf80ef

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/StatusBarIconView.java