Add flag indicating transport is device-to-device transfer

This flag allows BackupAgent instances to optionally backup more data
for a device-to-device transfer. i.e., an app developer might want to
only backup certain sensitive data if the transport is either
client-side encrypted or a direct device-to-device transfer.

Test: none, as it is just adding a flag
Bug: 72486408
Change-Id: I7512ed0b1510f48789fe47cb8b7379674c315e76