floss: Update connection info

The connection state exposes whether a device is paired (i.e. connection
is encrypted) so update BtConnectionState to reflect this information
and expose it via dbus.

Also adds the following helper apis to dbus:
* GetRemoteConnected -> bool
* GetConnectedDevices -> Vec<BluetoothDevice>

Bug: 220387308
Tag: #floss
Test: In btclient, try `list connected` and `device info`
Change-Id: I02cac6514fcc426511b8ac13c284a6e14ceb3a3f