[Multi-user] Make AccountSettingsBackupHelper multi-user aware

Bug: 123349308
Test: 1) atest RunBackupFrameworksServicesRoboTests
         2) atest CtsBackupTestCases
         3) atest CtsBackupHostTestCases
         4) atest GtsBackupTestCases
         5) atest GtsBackupHostTestCases

Manual testing (secondary user):
  1. Go to Settings -> Accounts -> [google-account] -> Account Sync,
     choose items to be synced
  2. adb shell bmgr --user [user-id] backupnow android
  3. Disable all itmes in sync settings
  4. adb shell bmgr --user [user-id] restore 1 android
  5. Verify the sync settings state was restored

Change-Id: I41b92cea54542b5311e1c6425521ab647bc9a787