Clear calling identity in BMS.backupNow

BMS.backupNow is called from GMSCore, which has a different calling
identity than the framework. This causes an IllegalArgumentException due
to uid mismatch when backupNow tries to schedule a job.

This occurs when the following conditions are combined:
1) Battery saver mode enabled
2) Network change detected
3) Backup pass is scheduled for now

Bug: 79441902
Test: 1) m -j RunFrameworksServicesRoboTests
2) Manual: Enabled battery saver, modified code to run backupNow with
each network change and overwrite previously scheduled KeyValueJob.
Then, change wifi to trigger scheduling the KeyValueJob.
Verified:
- IllegalStateException b/c of uid mismatch without change
- No exception and correct calling uid with change

Change-Id: Iac90cd435e3fc32ff5428236aa15507b36aa833d