API to select backup transport

This cl adds an API to select a backup
transport by its component name and
receive a callback when BackupManager
is bound to the transport. Calling this API
will make BackupManager bind to the transport
if it isn't already bound to it.

Also fixes the issue where BackupManager would
detect only one transport per package.

Ref: go/backup-transport-switching

Bug: 33616220
Test: Manually tested. GTS tests will be put up shortly.

Change-Id: I8c23bdbb84ceb05eb1fad9b3a8b9c4441cb06c74