Acquire the backup wakelock with a timeout

There have been a few reports of the backup service failing to call
release() on the wakelock and draining battery for hours. These are rare
failures and hard to debug individually but with the timeout we will
limit the worst case significantly.

I am setting the default timeout to 30 minutes. This should be more than
enough time for a backup or restore operation. If in the future these
operation get longer, the timeout is configurable by a setting.

Test: atest CtsBackupTestCases
Fixes: 364931501
Fixes: 357769443
Fixes: 289789401
Flag: EXEMPT bugfix
Change-Id: Iaac0218d5258031b6f67c3c56c776ce098e8a8c4