Filter on or prefer paired devices

[reillyeon]

As raised on the mailing list it would be helpful in some scenarios to be able to filter on whether a device is paired with the host device, or at least prefer them in the prompt shown by the navigator.bluetooth.requestDevice() method. The benefit of this is that it makes starting a new scan unnecessary and presents the user with a list of devices they are likely more familiar with since they are ones that have been paired previously rather than showing any device that happens to be in range.

[dlech]

Related: #511, #295