base

Add AppTransitionListener

Introduces the concept of a listener to be notified about app
transition events. The only client at the moment is window manager
which notifies activity manager about completed transitions, but this
can be used for various clients, including the status bar.

Bug: 19233606
Change-Id: Ia6fec5837b6eb4db90f3cb1c999d3f157ba6dd4a