Don't start prefetch jobs for TOP apps.

If a prefetch job hasn't started by the time an app is opened, then we
decide not to run it until the app is closed again. The rationale being
that "prefetch" is to do something before the app launch and it wouldn't
be useful while the app is on TOP. Any prefetch job that is already
running when the app is launched will be allowed to finish.

Bug: 194532703
Test: atest FrameworksMockingServicesTests:PrefetchControllerTest
Change-Id: I485cd0170a0066a82ab66bf92e36206a9a78bd78