Refactor restore to deal with heterogeneous datasets

Transport-based restore now handles both key/value and full-data
(stream) data delivery.

Also: PMBA now holds metadata for *all* apps, not just those with
backup agents.  Since we need to consult this for every restore-
at-install operation we cache this locally now, tagged per transport
and per remote dataset, to avoid having to re-download it as part
of every future restore operation.

Also fixed a bug in LocalTransport that was preventing restore of
key/value datasets, i.e. all of them that were nominally available
prior to this change.

NOTE: at present there is no automatic full-data backup; if for
testing purposes you need to create some to then use for restore,
you still need to use 'bmgr fullbackup ...' to push them.

NOTE: at present the unified transport restore uses a refactored
"engine" implementation to handle stream data that encapsulates
the existing "adb restore" implementation.  However, the adb
restore code path has not yet been refactored to wrap the newly-
extracted engine version; it still contains its own copy of all
the relevant logic.  This will change in a future CL, at which
point offline/USB archive restore will simply wrap the same
shared stream-restore engine implementation.

Bug 15330073
Bug 15989617

Change-Id: Ieedb18fd7836ad31ba24656ec9feaaf69e164af8