If a root process other than init calls __system_property_add, which
it should never do, it will break the design assumption that there is
only one mutator.
Pass O_EXCL to open() in map_prop_region_rw to ensure that only one
process ever has the property pages open for write.
(cherry picked from commit fb9b7b436f3ef94385f1b0c55ab81f246f0d96b8)
Change-Id: I6df3afedbfb5d07891b095aa24b78278381a5aaf
1d36ee1a6e