dumpstate: add the property(ro.radio.log_loc) for modem_logs

For temporary solution for not saving modem logs in bugreport,
I created the property(ro.radio.log_loc) to indicate the place
where modem_logs are saved.

During transition to hal dump of telephony, we will keep the property in
device configuration.
Bug: 33820081

Change-Id: I7712ce095686a54ea7f097f190611aec7a64d79c
Signed-off-by: Ecco Park <eccopark@google.com>