BackupManager#cancelBackups() API

Introduces a cancelBackups() API for
BackupManager. When this function returns,
it is guaranteed that currently running
backup operations won't interact with
the active transport.

Bug: 34760860

Ref: https://docs.google.com/document/d/18MnfwkDfKNtXQBPRmL8vpVgfLgSWJsDja1Nm1QV5hOw/edit#heading=h.9p6yo0wx44k3

Test: GTS tests at ag/1893365

Change-Id: I67f78699bbe763ea71c85937fbc01a5b48694eed