Add a "clear backed-up data" method to the backup mechanism

It's now possible to ask that the backup manager wipe the saved data for a given
application from the backing store.  LocalTransport implements this now but the
Google backend does not yet.  When the data is wiped, the on-device backup state
is also wiped to ensure that the next backup pushes all necessary data.

Bmgr has not yet been modified to actually call into this method, but it will
be soon.