Add ownership for BackgroundActivityStartController.

BackgroundActivityStartController is a new class to contain the logic
to permit activity launches from the background. The logic itself has
just been moved from ActivityStarter without any changes.

Bug: 236234252
Test: atest ActivityStarterTests
Change-Id: Id2f6b6a8bc7e8f6fbf206aa1b82f1a75d7e35aaa
Merged-In: I6ee0e839a4000c9e27630c2e654d945bf45b21e6