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.