Make IBackupTransport AIDL async

Make the transport AIDL async and introduce a callback AIDL for passing
back results. A follow-up CL will implement the client-side portion of
the callback class and update BackupTransportClient to use it.

Bug: 202716271
Test: m -j
Change-Id: I79cb376bd2a805bf2739470a3f538207ae6bd9a0
3 files changed