Always synchronize the OemUnlockAllowed bit to the FRP partition

ag/3023334 tried to synchronize the OemUnlockAllowed bit to the FRP
partition when deice is being unlocked.
However it missed a case when device was unlocked before ag/3023334
landed.
This CL will sync synchronize the OemUnlockAllowed bit on
isOemUnlockAllowed(), and it avoids redundant write by checking if
the bit is already synced.

Bug: 67043266
Bug: 68245782
Test: do factory reset on a device unlocked before ag/3023334 and FRP is
      not enforced
Change-Id: If209757cfbb202e180a5b2c1b5259af6d00aa599