Add "No New Data" flag for Key/Value Backups

Currently we have no method of telling transports that we have
attempted to make a Key/Value backup, but there was no data to
back up. This means that for apps with K/V backups their last
backup timestamp becomes older and older causing confusion for
users about whether the app data hasn't changed or whether the
app is failing to back up.

By introducing this flag we create a signal for the framework
to tell transports that, if there was any changed data, the
data would have been backed up, but the apps has not indicated
any changes have been made. This allows us to provide a UX
based on last non-erroring attempt rather than last data
change.

Bug: 147481066
Test: No code introduced in the CL.
Change-Id: Id832c7173da6341c6dc7b77fb72c289f2e1da624