Showing notification when activity is in PiP.

- Adding callback to SystemUI to be notified which package entered PiP,
  and when it leaves PiP.
- Showing a BTW notification on a new PiP-specific channel.

Bug: 36070720
Test: Launch PiP, observe notification, leave PiP, observe no notification
Change-Id: Ibe10298288fe0464d7d136f2571e855f3f5c70ea