Settings: Lost cached BT devices in found devices list

Turn off BT and then turn it on again, it will lose
some devices in the list.

This is because it doesn't add cached device into list,
but just rebuild it. We nedd to add them into the list again.

Change-Id: I2dadf9ddec045f3bd7cf7a04b9b0dc024e536530
CRs-Fixed: 1093008