Add basic prefetch job handling.

1. Add a PrefetController that will mark prefetch jobs as ineligible
   to run if the system doesn't think the app will be launched in the
   next few hours.
2. Force batch all prefetch jobs unless the system thinks the app will
   be launched within the next hour.

Upcoming:
* Adding functionality in UsageStats to determine the next time the app
  will be launched.

Bug: 194532703
Test: atest FrameworksMockingServicesTests:PrefetchControllerTest
Change-Id: Ic4e911eb53b4cfb5a904eec0aa59fdc80f029cb1