Label /oem files

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.

Bug: 20816563
Change-Id: Ibe8d9ff626eace8a2d5d02c3f06290105baa59fe

file_contexts

@@ -185,9 +185,11 @@
 /vendor/bin/gpsd	u:object_r:gpsd_exec:s0
 
 #############################
-# ODM files
+# OEM and ODM files
 #
-/odm(/.*)?		u:object_r:system_file:s0
+/odm(/.*)?              u:object_r:system_file:s0
+/oem(/.*)?              u:object_r:oemfs:s0
+
 
 #############################
 # Data files