Bluetooth keyboards require the host to display a passkey for the user
to type. Previously this was not supported because the agent capability
was set to DisplayYesNo and the DisplayPasskey/DisplayPinCode events
were not handled.
- Change agent capability to KeyboardDisplay
- Add DisplayPasskey dialog with typed-key progress indicator
- Add DisplayPinCode dialog for older keyboards
- Auto-dismiss display dialogs when device reports Paired=true
