Fix for Bluetooth device name is resetting to default name after reboot

Reason: When the BT device name is updated is getting saved to the
config data pointer but not to the persistent data (i.e. bt_config.conf).
So, when the reboot is happening it is not able to get the updated device
name from the persistent data (i.e. bt_config.conf) as during reboot
bt_config_flush is not called.

Fix: Saving the BT Device name to persistent data using btif_config_flush
once it is set.

Test: Manual Reboot test
Bug: 110301897
Bug: 120631885
Change-Id: Ie329b475eaaf208ed667f27e271a00e230e4b95c
Signed-off-by: Aiswarya Cyriac <aiswarya.cyriac@intel.com>
Signed-off-by: Gaganpreet kaur <gaganpreetx.kaur@intel.com>