HALO: Option to pause the underlying app (1/2)

- adds an option to pause the active app when using HALO
- this is enabled by default for devices with low memory
- (currently: <640MB RAM available to the kernel)
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.
Merge branch 'jellybean' of https://github.com/UltimatumKang/android_frameworks_base into jellybean
Revert "Merge scrollfixthing"

This reverts commit cb3344f691d464ad99da83a1934d6e66a9b903e0.