Add more instrumentation to BackupManager.

This CL continues work of: ag/1835775 and ag/1870141 adding a few more
logging constans described here:
https://docs.google.com/document/d/1sUboR28LjkT1wRXOwVOV3tLo0qisiCvzxIGmzCVEjbI/edit#

Test: manual

BUG: 34873525
Change-Id: I8c81d8cfea5538e89de94919062eeff6ae09dce4
3 files changed