Extracts core logic of ActivityView into TaskEmbedder

TaskEmbedder is a new building block for task embedding which
will be used by SystemUI to perform more complex operations
than what can be acheived using ActivityView today.

For task embedding use cases, integrating TaskEmbedder directly
with a SurfaceView (or other surface) will allow:

- management of multiple tasks within a single parent surface
- access to the surfacecontrol of each task for manipulation/animation
- (SurfaceView) configure whether zOrder above or below.

See: go/av-refactor

Change-Id: Ia813c52bc2da3a776e727b5bbd2b03b8ff09f302