Remove Global Leakage of winrt namespaces

We have code today in shared headers that looks like this:

namespace winrt {
using namespace Windows::UI::Core;
using namespace Windows::Foundation;
using namespace Windows::Foundation::Collections;
} // namespace winrt

This type of global leakage can lead to confusion of where namespaces are coming from, can leak to customers, and has bitten us in Office before, where incorrect types are silently included. E.g. the wrong IInspectable.

Cppcoreguidelines, which we try to follow explicitly calls this out as something not to do http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rs-using-directive

We should remove this, ensure it's not leaking to customers, and make namespace reduction explicit.

Start by locating the shared headers containing the shown winrt using-directives and inspect how their namespace names are exposed to customers. Remove the global leakage and make namespace reduction explicit, then verify that affected code still resolves the intended types without silently selecting incorrect ones.