Unsubscribe listeners when stopping UserBackupManagerService and remove unused code

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

Also cleanup by removing unused code:
a) mBackupThread from BackupManagerService
b) TAG from BackupAgentTimeoutParameters
c) getRunningFullBackupTask from UserBackupManagerService

Bug: 135261178
Bug: 135180752

Test: 1. atest -v RunBackupFrameworksServicesRoboTests
2. atest -v $(find frameworks/base/services/tests/servicestests/src/com/android/server/backup -name '\''*Test.java'\'')
3. atest -v CtsBackupTestCases
4. atest -v CtsBackupHostTestCases

Change-Id: Ie199dd49c336a2519c0f8e6c8dda12d8cba8350a