Allow scanning extractor library directory

so we can dlopen the libraries that are there

Test: build&run

Merged-Id: Ia1fa1fd65295cffe6c8a3d31db53bd3339a71855
Change-Id: Ia1fa1fd65295cffe6c8a3d31db53bd3339a71855

public/mediaextractor.te

@@ -34,6 +34,9 @@ allow mediaextractor apk_data_file:file { read getattr };
 allow mediaextractor asec_apk_file:file { read getattr };
 allow mediaextractor ringtone_file:file { read getattr };
 
+# scan extractor library directory to dynamically load extractors
+allow mediaextractor system_file:dir { read open };
+
 ###
 ### neverallow rules
 ###