Improve notification shade collapse code.

- Collapse after dismissing the last notification
- Avoid duplicate code paths for collapsing
- Improve variable naming according to AOSP review suggestions

Change-Id: Ic6f26a61f263c5beebbcc9725fe8914d3858576f
Make notification-shade-collapse-after-dismiss behaviour configurable.

Change-Id: I1be0be2753b479e6d2e4fc8e17d971d03ce01bb7
Merge "Improve notification shade collapse code." into cm-10.2
Merge "Make notification-shade-collapse-after-dismiss behaviour configurable." into cm-10.2