sepolicy : add allow rule to wcnss_service to write to sdcard

ptt socket app service will redirect logs received by wlan driver
and firmware to sdcard location where it needs lnk file read access
to point to sdcard/mem physcial location  and  write access to sdcard.

Link access is provide, as /sdcard is a symbolic link of
/storage/self/primary/

These rules are in user_debug as this is not needed in production builds.

CRs-Fixed: 901039
Change-Id: I8917a7edd4da291ddc07e1aa0894fd411c1e1d46