Bluetooth: GAP: Call system.exit in cleanup

As we are not clearing the internal static memory during bt off,
paired device list will be updated from internal memory
instead of bt_config.xml and paired list will be different for
different user logins.
If we call system exit it will kill the process and clean intenal
memory

Change-Id: I69b5485e2202413692c685a595de9ce96985f669
CRs-Fixed: 798825