fix: handle preview issues

[rosen-vladimirov]

fix: remove debug qr code message

fix: do not start multiple watchers …

In some cases (for example tns preview and scan QR code simultaneously with several devices) we start multiple watchers per platform. We should always have single watcher per platform (one from webpack and one native in fact).

fix: handle preview initial sync once per device …

Handle preview initial sync once per device (in some cases PubNub reports the device multiple times). Also prepare all the files once per platform, not per each device.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

What is the new behavior?

Fixes/Implements/Closes #[Issue Number].

[rosen-vladimirov]

test cli-preview cli-smoke cli-templates

[rosen-vladimirov]

test cli-smoke cli-preview tns-core-modules#rc nativescript-dev-webpack#rc android#rc ios#rc tns-android#rc tns-ios#rc

[rosen-vladimirov]

test cli-templates tns-core-modules#rc nativescript-dev-webpack#rc android#rc ios#rc tns-android#rc tns-ios#rc