Add monitoring to backup in BackupManager.

This is the first CL of many that will add instumentation to
backup/restore operation in the BackupManager. For more details please
point to:
https://docs.google.com/document/d/1sUboR28LjkT1wRXOwVOV3tLo0qisiCvzxIGmzCVEjbI/edit#
This first Cl introduces 3 events that we sent to the monitor.

Test: ag/1858962 (same topic)

BUG: 34873525

Change-Id: I6c338b6fd9f4d7c8670dac201897250b6b170677