Properly update config when the MAC Address changes

The existing code has two issues:
- We pass a 0 size value for the buffer we intend to contain the value
we're getting. This causes btif_config_get_str to always return
positively but never write a value because its usage of srtlcpy doesn't
think it has space to write.
- We check to see if the address in the config is the same as the one
received from the controller using (strcmp(...) == 0). However, strcmp
returns 0 if they are the same. It will be a non-zero value if they are
different.

Following this change, the bt_config.conf file should have the correct
value for the MAC Address in the case the controller interface starts
reporting something new.

Bug: 169686820
Test: setprop persist.vendor.service.bdroid.bdaddr <something new>; cat
/data/misc/bluedroid/bt_config.conf | grep Address;

Change-Id: I6cdb70776fef689426baf58be0ccaf4abe9c9023