Read selinux_denial_metadata from /vendor

selinux_denial_metadata contains device-specific information, so it is
being moved to /vendor.

Bug: 5159394
Test: bug information is still preserved in avc logs, e.g.
audit(0.0:248): avc: denied { read } for
name="u:object_r:vendor_default_prop:s0" dev="tmpfs" ino=18012
scontext=u:r:platform_app:s0:c512,c768
tcontext=u:object_r:vendor_default_prop:s0 tclass=file permissive=0
b/79617173 app=com.android.systemui
Change-Id: I23293e715b5d793a54466a128ccd57f44370cf5a