KV] Don't set current token if no data was moved

Bug: 119299848
Test:
1) atest KeyValueBackupTaskTest
2) Manual:
 - Run 'adb shell bmgr init' for the active transport to wipe all backup data
 - Run 'adb shell dumpsys backup' and verify string 'Current: 0', i.e. the current token is
   set to 0
 - Create a test app implementing a BackupAgent that writes no data in onBackup() and
   install it on the device
 - Run 'adb shell bmgr backupnow --non-incremental <test_app_package>' to initiate a backup,
   where --non-incremental flag makes sure PM is not added to the backup queue
 - Run 'adb shell dumpsys backup' and verify string 'Current: 0' again

Change-Id: I595bea9874fd84d0c81b32a509c970a1b142872c