installd: r_dir_file(installd, system_file)

Allow installd to read through files, directories, and symlinks
on /system. This is needed to support installd using files in
/system/app and /system/priv-app

Addresses the following auditallow spam:

avc: granted { getattr } for comm="installd"
path="/system/app/Bluetooth/lib/arm/libbluetooth_jni.so"
dev="mmcblk0p41" ino=19 scontext=u:r:installd:s0
tcontext=u:object_r:system_file:s0 tclass=lnk_file

avc: granted { getattr } for comm="installd"
path="/system/priv-app/MtpDocumentsProvider/lib/arm64/libappfuse_jni.so"
dev="dm-0" ino=2305 scontext=u:r:installd:s0
tcontext=u:object_r:system_file:s0 tclass=lnk_file

avc: granted { read open } for comm="installd"
path="/system/priv-app/TelephonyProvider" dev="mmcblk0p43" ino=1839
scontext=u:r:installd:s0 tcontext=u:object_r:system_file:s0 tclass=dir

avc: granted { read } for comm="installd" name="Velvet" dev="mmcblk0p43"
ino=1841 scontext=u:r:installd:s0 tcontext=u:object_r:system_file:s0
tclass=dir

avc: granted { read open } for comm="installd"
path="/system/priv-app/GoogleOneTimeInitializer" dev="mmcblk0p43"
ino=1778 scontext=u:r:installd:s0 tcontext=u:object_r:system_file:s0
tclass=dir

avc: granted { read open } for comm="installd"
path="/system/app/PlayAutoInstallConfig" dev="mmcblk0p43" ino=112
scontext=u:r:installd:s0 tcontext=u:object_r:system_file:s0 tclass=dir

Test: policy compiles
Change-Id: I5d14ea2cd7d281f949d0651b9723d5b7fae2e1f2