Cancel any running backups in backup tests setUp().

`backupnow` returns immediately if a backup is already running. Scheduled backup jobs on wear which may execute at anytime, will interfere with the test executions and cause flakiness.

Test logs attached to buganizer ticket.

Test: atest CtsBackupTestCases
Flag: TEST_ONLY
Bug: 402423383
Change-Id: I3a0247e817e0ae8af05646dd4a9699503284ce0c