Use separate message ids for backup and restore operations

This cl is part of a group of CLs to support
some parallelism in BackupManager's operations.
Separate message ids are used for handler messages
for backup and restore operations so that both
operations can run parallely.

Bug: 29010684
Test: Manually tested by installing a test backup agent that times out.
Change-Id: I57a5d063fb4446e610af81ac6f039d3fc8414a46