Offload the mRunningFullBackupTask.handleCancel() call from the main

thread to another thread

The system_server process was being killed by Watchdog, as the main
thread was waiting for the mCancelLock for over a minute.

Bug: 35968123
Test: make gts -j40 && gts-tradefed run gts -m GtsGmscoreHostTestCases -t com.google.android.gts.backup.BackupManagerHostTest
Change-Id: Ia146569d2c741b35a6f6c9bfc4c5ddf8539b6242