Log storage savings info with the GlobalHibernatedApps atom

Retrieve the number of freed bytes from PM and store it in
GlobalLevelState when a package is globally hibernated. Reset the value
when it gets unhibernated and write it to disk to persist across device
reboot.

Bug: 188819665
Test: atest AppHibernationServiceTest
Change-Id: I187ec803776cc76c77a660e1ba2f72c2e77765f9