Enable runtime turndown of backup/restore services

The heavy implementation of the backup manager service is now sitting
behind a lightweight trampoline that actually provides the binder
call interface.  The indirection allows us now to tear down the
implementation on the fly without breaking callers who have cached
binder references to the backup services: these callers will simply
see their future invocations failing benignly.

In addition there is now an API for suitably privileged callers such
as device policy management to effect this turndown.

Finally, there is now a static system property, "ro.backup.disable",
that a product can use to outright remove backup/restore operation
from the system's operation.  The public APIs will continue to be
safely usable on such products but no data will be moved to or
from the device.

Bug 17367491

Change-Id: I8108e386ef3b5c967938fae483366d6978fe4e04