Allow drmserver and mediaserver to read apk files.

Required to support passing resources via open apk files over Binder.
Resolves denials such as:
 avc:  denied  { read } for  pid=31457 comm="SoundPoolThread" path="/mnt/asec/au.com.shiftyjelly.pocketcasts-1/pkg.apk" dev="dm-10" ino=12 scontext=u:r:mediaserver:s0 tcontext=u:object_r:asec_apk_file:s0 tclass=file
 avc:  denied  { read } for  pid=31439 comm="Binder_2" path="/mnt/asec/au.com.shiftyjelly.pocketcasts-1/pkg.apk" dev="dm-10" ino=12 scontext=u:r:drmserver:s0 tcontext=u:object_r:asec_apk_file:s0 tclass=file

We do not allow open as it is not required (i.e. the files
are passed as open files over Binder or local socket and opened by the
client).

Change-Id: Ib0941df1e9aac8d20621a356d2d212b98471abbc
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>