file_contexts

Files on the /oem partition are weird. The /oem partition is an ext4
partition, built in the Android tree using the "oem_image" build target
added in build/ commit b8888432f0bc0706d5e00e971dde3ac2e986f2af. Since
it's an ext4 image, it requires SELinux labels to be defined at build
time. However, the partition is mounted using context=u:object_r:oemfs:s0,
which ignores the labels on the filesystem.

Assign all the files on the /oem image to be oemfs, which is consistent
with how they'll be mounted when /oem is mounted.

Other options would be to use an "unlabeled" label, or try to fix the
build system to not require SELinux labels for /oem images.

(cherrypicked from commit 2025fd14)

Bug: 20816563
Change-Id: Ibe8d9ff626eace8a2d5d02c3f06290105baa59fe