Notify transports of empty K/V backups

In order to allow transports to know when a K/V backup would have
been performed, but was not due to no data changes being reported,
we need to introduce some code to find the K/V backup participants,
remove any which are backed up in this sweep or are currently failing
to back up due to an error, and then inform the transport of the
remaing set.

This CL introduces the code to do that.

We use a state file to determine if a package backed up without an
error on the last run, if the backup fails we remove the file for
that package. When all packages with changed data have been backed
up we get a list of all packages which have the file set (i.e. were
successful), remove the set of packages backed up on this run, and
inform the transport the rest had no data changes.

Bug:147481066
Test: m -j RunBackupFrameworksServicesRoboTests
Change-Id: I5c9f94c925096faf7b65307c0be1a7aba48c1cfb