sepolicy: allow vold to create files on external sdcard

This is required for ASEC support. Vold can already create and
access directories, but do not yet have the permission for files.

Change-Id: I5082bbff692e5dc53c7000e4b3a293e42d33f901
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