Fix the file permissions of /data/misc/bluedroid/bt_config.conf

Fix the file access permissions and group ownership of
"/data/misc/bluedroid/bt_config.conf" so the file can be reused when
switching users on the device.
For that purpose, we need to do the following:

1. Set the set-group-ID (bit 02000) flag for directory "/data/misc/bluedroid"
   so the files created in that directory will have group-id of
   "net_bt_stack" .

2. Change the file's permissions of file "/data/misc/bluedroid/bt_config.conf"
   to Read/Write by User and Group.

Bug: 21493919
Change-Id: I48b73711789734b5221231729f8cd9ff9bb94be7