Consistent "low storage" behavior.

When freeing cached data, the caller can now provide a "reserved"
size which we won't clear cached data from.

Bug: 38008706
Test: cts-tradefed run commandAndExit cts-dev -m CtsJobSchedulerTestCases -t android.jobscheduler.cts.StorageConstraintTest
Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Change-Id: Ieb91b3e5345a950d4785fd7915f520f0a68a567a