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