BackupAgent#getBackupQuota() API

This cl adds a getBackupQuota() method
to the BackupAgent abstract class. This
can be used by the agent to determine
how much data it can back up.

Ref: https://docs.google.com/document/d/18MnfwkDfKNtXQBPRmL8vpVgfLgSWJsDja1Nm1QV5hOw/edit#heading=h.unncio2r7de2

Bug: 33750400

Test: GTS test at ag/1850525

Change-Id: I2eb5c0066c99732ec84cae9211f3a1edaad2baf1