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
Ignore-AOSP-First: Internal merge conflict, AOSP change in aosp/2169485
Change-Id: I6ee0e839a4000c9e27630c2e654d945bf45b21e6