Move BackgroundActivityStartMode to ActivityOptions

The ComponentOptions class is not publicly accessible, which causes
broken links in the documentation and in general confusion with
automatic imports in the IDE.
This is technically an API change, but practically the @IndDef is only
retained in the source and developers cannot access the definitions in
the hidden ComponentOptions class.

Test: atest ActivityStarterTests
Bug: 326324768
Flag: NONE
Change-Id: I6101c0582474dd8ec9b0fcdd81fc1183c6a3ddb0