vold_prepare_subdirs: grant chown

Addresses:
avc: denied { chown } for comm="vold_prepare_su" capability=0
scontext=u:r:vold_prepare_subdirs:s0
tcontext=u:r:vold_prepare_subdirs:s0 tclass=capability

Bug: 71796118
Test: build
Change-Id: I64b2f1ad8d6e0748c5820b8a37a4fc4f4101d1fb