Check backup wakelock is held before releasing

This is to prevent a rare crash that can happen if something releases
the wakelock beforehand. I'm not sure exactly what sequence of events
leads to the crash scenario but this seems like a sensible check
to have anyway since we have multiple operations that can
acquire/release the wakelock.

Flag: EXEMPT bugfix
Fixes: 349280741
Test: CtsBackupTestCases, CtsBackupHostTestCases
Change-Id: I4a5c7ea0cf381de051a4a4afdf2426f9715515c8