add mediatranscoding to apex file context

bug: 159172726
Change-Id: I8fbedab2605167af637108d5f564abf6c78c32bd
diff --git a/apex/com.android.media-file_contexts b/apex/com.android.media-file_contexts
index f6b21da..8822046 100644
--- a/apex/com.android.media-file_contexts
+++ b/apex/com.android.media-file_contexts
@@ -1,2 +1,3 @@
 (/.*)?                u:object_r:system_file:s0
 /lib(64)?(/.*)        u:object_r:system_lib_file:s0
+/bin/mediatranscoding     u:object_r:mediatranscoding_exec:s0