Allow mediaprovider to search /mnt/media_rw

Mtp needs access to this path in order to
change files on an sdcard.

Fixes denial:

05-14 17:40:58.803  3004  3004 W MtpServer: type=1400 audit(0.0:46):
avc: denied { search } for name="media_rw" dev="tmpfs" ino=10113
scontext=u:r:mediaprovider:s0:c512,c768
tcontext=u:object_r:mnt_media_rw_file:s0 tclass=dir permissive=0
b/77925342 app=com.android.providers.media

Bug: 77849654
Test: no denials using mtp with emulated sdcard
Change-Id: I27b5294fa211bb1eff6d011638b5fdc90334bc80

prebuilts/api/28.0/private/mediaprovider.te

@@ -19,6 +19,8 @@ allow mediaprovider cache_file:lnk_file r_file_perms;
 dontaudit mediaprovider cache_private_backup_file:dir getattr;
 dontaudit mediaprovider cache_recovery_file:dir getattr;
 
+# Access external sdcards through /mnt/media_rw
+allow mediaprovider { mnt_media_rw_file }:dir search;
 
 allow mediaprovider app_api_service:service_manager find;
 allow mediaprovider audioserver_service:service_manager find;

private/mediaprovider.te

@@ -19,6 +19,8 @@ allow mediaprovider cache_file:lnk_file r_file_perms;
 dontaudit mediaprovider cache_private_backup_file:dir getattr;
 dontaudit mediaprovider cache_recovery_file:dir getattr;
 
+# Access external sdcards through /mnt/media_rw
+allow mediaprovider { mnt_media_rw_file }:dir search;
 
 allow mediaprovider app_api_service:service_manager find;
 allow mediaprovider audioserver_service:service_manager find;