Let vold_prepare_subdirs completely clean deleted user data.

After adding a new user, deleting it, and rebooting, some of the user's data still remained.  This adds the SELinux permissions necessary to remove all of the data.  It fixes the followign denials:

avc: denied { rmdir } for scontext=u:r:vold_prepare_subdirs:s0 tcontext=u:object_r:system_data_file:s0 tclass=dir
avc: denied { unlink } for scontext=u:r:vold_prepare_subdirs:s0 tcontext=u:object_r:system_data_file:s0 tclass=file

Bug: 74866238
Test: Create user, delete user, reboot user, see no denials or
leftover data.

Change-Id: Ibc43bd2552b388a9708bf781b5ad206f21df62dc