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

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