PIP: Fix crash when the PIP menu activity is started by intent

This also cleans up unused PIP activities from the Manifest file.

Bug: 37912313
Test: Manual test as follows
  - Checked that my test app crashes when it sends intent to start
    the PIP menu.
  - Checked that starting the PIP menu activity with the am command
    doesn't cause System UI to be crash.
Change-Id: I3a8171fcf6017c2616b8e83ab8a281b5d7e56ab7