feat(preview-api): expose api for getting logs from preview app

[Fatme]

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What is the current behavior?

No api for getting logs from preview app

What is the new behavior?

tns.previewDevicesService.on("deviceLogData", (deviceId, log) => {
    console.log(`received log ${log} from device: ${deviceId}`);
});

[DimitarTachev]

Isn't it better to add the DEVICE_LOG_EVENT_NAME string in the previewAppLogProvider itself instead of using the constants. In this way it will be also available in the public API and we will not require a hardcoded string (deviceLogData). Also the data suffix in the event name is a little bit strange. It could be just "deviceLog"