Consolidate Responsible API OWNERS

Introduces a new file that defines the owners for BAL logic.

Use this file for background activity start related files:
- BackgroundActivityStartController
- BackgroundActivityStartCallback
- BackgroundStartPrivileges
- BackgroundStartPrivilegesTest (in tests)

Change-Id: I3315a0f2216ed22a28b952f30afe0c3d14cfc955
Test: owners change only