Split checks for visibility and other exemptions

Preparation to limit checks to visible only if
MODE_BACKGROUND_ACTIVITY_STARTS_ALLOW_ALWAYS is selected. This only
splits the visibility based exemptions into separate methods.

Flag: EXEMPT refactor
Bug: 352182359
Test: atest BackgroundActivityLaunchTest
Change-Id: Idb69119a5a93bbfef85061d0289423a079475eb3