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