Fix little derp causing visual bug

To reproduce, disable halo, and press "clear all notifications" button

Change-Id: Ice7ab62229e27db53ef51fd5df55dbcd0a86358b
Reworked transparent layer
When a halo'ed window is opened, the transparent layer will be always visible. It will manage the background dim.
This fixes some apps dimming its sandboxed processes (like chrome).

LayerActivity manages its own lifecycle. When it's focused after paused it finishes itself.

System processes (identified by "android" pkg in SystemUI and system uid in Activity) don't use the layer and still use their own background. This
has been kept this way to not interfere with their activity.

Conflicts:

	packages/SystemUI/src/com/android/systemui/Transparent.java
	packages/SystemUI/src/com/android/systemui/statusbar/BaseStatusBar.java
LayerActivity: Corrected lifecycle
Should contemplate the case that the activity gets stopped