Respect the transport's requestFullBackupTime() backoff

We now make sure to pause by at least requestFullBackupTime() between full-data
backup operations, to give the transport the ability to apply traffic control
while we're running the queue of eligible packages.

Also, we now reset a package's queue position whenever a full-data backup for
that package is run explicitly via adb.

Bug 19732890

Change-Id: I6cf24495ad18eebd55557f229d11c703e5b7f529