sepolicy: allow vold to mount ext4 sdcard

When vold mounts an ext4 sdcard, it needs to force the context to
sdcard_external.

avc:  denied  { relabelfrom } for  pid=190 comm=vold scontext=u:r:vold:s0 tcontext=u:object_r:labeledfs:s0 tclass=filesystem
avc:  denied  { relabelto } for  pid=190 comm=vold scontext=u:r:vold:s0 tcontext=u:object_r:sdcard_external:s0 tclass=filesystem
avc:  denied  { relabelfrom } for  pid=190 comm=vold scontext=u:r:vold:s0 tcontext=u:object_r:sdcard_external:s0 tclass=filesystem

Change-Id: I80f42fbdf738dee10958ce1bdc1893a41234f0d9