Agile factories are cached

[jonwis]

@oldnewthing points out that this:

While this model reduces the idiomatic elegance of C++/WinRT’s projection, it greatly reduces binary footprint. Instead of each “.Create()” being a fetch of the property cache, an addref of it, a call, and a releae, it’s a single call per line with the fetch-and-release applied once in the sequence.

... is not correct for agile factories; once the factory has been cached it's a "look at cache then call." Which still requires looking at the cache and calling, potentially also fetching the factory into the cache. See https://github.com/microsoft/cppwinrt/blob/bf4459b25aeeb7093e8262fb3b29ca70b0ab2e60/strings/base_activation.h#L393 for where it can skip the addref/release on the factory itself.