Floss: Adds get_discoverable_mode into BluetoothQA

This change adds get_discoverable_mode into BluetoothQA
interface and show it in the adapter status.

Bug: 278653204
Test: Manually test with btclient:
        adapter discoverable limited 60
        adapter show
Tag: #floss
Change-Id: I71b6e849d25e9689333bfb422ba4b9897ebf0ad8