Introduce API to get hibernation stats

Introduce an API to get relevant stats about hibernated apps. For now,
this only includes the amount of storage saved from the app being
hibernated. This will be used to in user-facing UI surfaces to show how
much storage hibernation has saved the user.

Bug: 213480570
Test: atest AppHibernationServiceTest
CTS-Coverage-Bug: 216383448
Change-Id: I41877aa2f4959b3f2b13731c8336077555379338