Allow modem diagnostic app to access default prop

log:
avc: denied { read } for name="u:object_r:default_prop:s0" dev="tmpfs" ino=154 scontext=u:r:modem_diagnostic_app:s0:c512,c768 tcontext=u:object_r:default_prop:s0 tclass=file permissive=0 app=com.google.mds

Bug: 222509956
Change-Id: I50302b38f074e3f1a078ee48896154353e0937b6