Implement ACTION_REQUEST_DISCOVERABLE to allow discovery from `discoverDevices`

[kenjdavidson]

Mobile Device Environment
On which device(s) would you like this feature to be implemented. If Android only, the IOS implementations must be created as well - Android ONLY

Is your feature request related to a problem? Please describe.
The library allows discoverDevices but does not allow placing the device in discoverable mode.

Describe the solution you'd like
Include functionality for making the device discoverable. This is the opposite to discoverDevices.
see https://developer.android.com/reference/android/bluetooth/BluetoothAdapter#ACTION_REQUEST_DISCOVERABLE