Only use Visual Studio Telemetry on VS 2017 and above

[jcansdale]

GitHub for Visual Studio is currently broken on Visual Studio 2015, so I'd like this get this PR out ASAP!

Make sure that Microsoft.VisualStudio.Telemetry is only used on Visual Studio 2017 and above.

What this PR does

• Factor out VisualStudioUsageTracker, an implementation of IUsageTracker that uses VS Telemetry
• Add option to UsageTracker to use VS Telemetry
• Only use VS Telemetry when running on Visual Studio 2017 and above

Reviewers

The aim of this PR was to factor out VisualStudioUsageTracker which can be selectively disabled in Visual Studio 2015. If we deprecate the GitHub telemetry in a future version, we can simply replace UsageTracker with VisualStudioUsageTracker. I've deliberately touched UsageTracker as little as possible because it's complex and I don't want to break anything!

I've tested this on Visual Studio 2015, 2017 and 2019 using the steps below.

How to test

Repro

1. Open Visual Studio 2015
2. Install GitHub for Visual Studio 2.10.6 from the Visual Studio Marketplace and Restart
   
3. Open Team Explorer - Connect
4. Click on Manage Connections > Connect to GitHub
5. Click on the GitHub tab (if GitHub Enterprise is selected)
6. Click Sign in with your browser
7. After authenticating return to Visual Studio
8. Click on the GitHub tab (if GitHub Enterprise is selected)
9. Expect the following error
   

Fix

1. Close all instances of Visual Studio and open GitHub.VisualStudio.vsix from this PR
   https://ci.appveyor.com/project/github-windows/visualstudio/builds/27765286/artifacts
2. Install for and open Visual Studio 2015
3. Open Team Explorer - Connect
4. Click on Manage Connections > Connect to GitHub
5. Click on the GitHub tab (if GitHub Enterprise is selected)
6. Click Sign in with your browser
7. Expect Connect to GitHub dialog to close

Fixes #2423

[alpaix]

LGTM