feat(preview): add public API for deviceFound and deviceLost for preview devices

[Fatme]

We want to expose deviceFound and deviceLost for preview devices as a public api so Sidekick can use them.

const tns = require("nativescript-cli");
tns.previewDevicesService.on("deviceFound", device => {
    console.log("device found", device);
});
tns.previewDevicesService.on("deviceLost", device => {
    console.log("device lost", device);
});

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[DimitarTachev]

Do we need to expose a public setter? We could expose just a getConnectedDevices method.

[DimitarTachev]

In the context of this service, this should be something like set/update ConnectedDevices.