Properly check for thread access on UWP

[mattleibow]

Description of Change

Fix the logic that determines whether or not on the UI thread.

Bugs Fixed

None.

API Changes

None.

Behavioral Changes

Previously, the check was CoreApplication.MainView.CoreWindow.Dispatcher == null for background threads, but this is incorrect. The window will always have a thread since it was created on a thread at some point. To check if the thread is the UI thread (or if the current thread is the same as the thread as the window), use the HasThreadAccess property.

PR Checklist

• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Updated documentation (see walkthrough)

[dend]

✅ Validation status: passed

File	Status	Preview URL	Details
Xamarin.Essentials/MainThread/MainThread.uwp.cs	✅Succeeded

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