Multiple concurrent dictionary lookups via a dictionary wrapped in a ReadOnlyDictionary are causing the concurrent exception.

[jlaanstra]

Describe the bug
Multiple concurrent dictionary lookup can result in the internal CSWinRT lookup caching to simultaneously insert and lookup from the same cache causing a concurrent modification exception being thrown.

To Reproduce
See task.ms/43496689

Expected behavior
No crash.

Version Info
CsWinRT 2.0.1

Additional context
See task.ms/43496689