Prevent writing to FRP partition during factory reset.

Avoid potential race condition between FRP wipe and write operations
during factory reset by making the FRP partition unwritable after
wipe.

Bug: 30352311
Test: manual

CVE-2017-0498

Change-Id: If3f024a1611366c0677a996705724458094fcfad
(cherry picked from commit a629c772)
(cherry picked from commit a9437bd1)
(cherry picked from commit 1c4d535d0806dbeb6d2fa5cea0373cbd9ab6d33b)