Unsubscribe listeners when stopping UserBackupManagerService

Unsubscribe listeners as otherwise they schedule work on a stopped handler thread
after the user backup manager service is stopped

This also ensures that any dangling wakelocks are released when they are
garbage collected

This was already done in master in ag/8036218

Test: atest CtsBackupTestCases CtsBackupHostTestCases
Test: m RunBackupFrameworksServicesRoboTests && atest RunBackupFrameworksServicesRoboTests

Test: blaze run -- //experimental/users/nathch/py/bug_repros:repro 136264323 -m w_codechange
followed by "adb shell kill -10 pid" (where pid = "adb shell pgrep system_server") to force GC
see in logs: PowerManager: WakeLock finalized while still held: *backup*-0

Test: blaze run -- //experimental/users/nathch/py/bug_repros:repro 136264323 -m backup_w_codechange
followed by same as above

Bug: 136264323
Bug: 135261178
Change-Id: I769d750a46faa3d28634d302e0f8e18cce41a744