Cleanup background activity starts for release.

This removes the parts of the feature that were only needed in beta
releases:
- Toast when a start is or would be blocked.
- Configurable whitelist of exempt apps.
- Global setting to disable.

Not to be submitted until after beta 5 is cut.

Bug: 131747138
Test: atest BackgroundActivityLaunchTest
Test: atest RootWindowContainerTests
Test: atest WmTests:ActivityStarterTests
Test: atest CtsWindowManagerDeviceTestCases:ActivityStarterTests
Test: atest CtsAppTestCases:.ServiceTest

Change-Id: I3d8b9c72c8fac86b93ac57a8a1988f4133043af8