Backup/Restore accounts - bugfix - DO NOT MERGE

* Followup to 85d765f4
* Workaround for (HTC bug: 2275383) & (Moto bug: 2226582)
* Restores mSyncKey as null instead of empty string, which is how
  a new account is initialized.  Bug: 2385980
* Cleanup synchronized logic in backup & restore
* Minor cleanups & improved comments

Cherry-picked from master d612717340b2670776a75a1d2e9e3df81fd4c052
5 files changed