Validate transport before selecting in UserBackupManagerService.

Make sure transport is registered before changing current transport in UserBackupManagerService.
If the input transport is not registered, throw an error to user.

Fixes: 163539637

Test: 1. atest -v BackupFrameworksServicesRoboTests
	2. Manual test on device.
		- bmgr list transports # which lists registered transports
		- Verify selecting transport succeeds on registered transports
		- Verify selecting transport throws an error on non-registered transports
Change-Id: I7e3572a13df07b2dee34e295ea07f1ccff5ca505