Navigationview: Fix init-once of the GlobalDependencyProperty

[lhecker]

This commit fixes a bug in NavigationView where a GlobalDependencyProperty
was repeatedly instantiated in the constructor. Using the atomicity
of statics we ensure it's treated as a singleton instead.

Description

Additionally this commit makes a minor change to use
references during loop iteration to avoid copies

Motivation and Context

Related to #6240, which introduced this code.
Closes #6760.

[StephenLPeters]

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[StephenLPeters]

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[lhecker]

Apparently you can't hash cppwinrt types. I guess you have to wrap them with a com_ptr<> first?
-> Change reverted.

BTW I've figured out why I couldn't build this project. This is ill-formed:

microsoft-ui-xaml/dev/inc/tracker_ref.h

Lines 142 to 145 in 6225742

	explicit tracker_ref(nullptr_t)
	{
	static_assert(false, "tracker_ref should only be used in ReferenceRuntimeClass-derived field members and 'this' should be passed for the owner");
	}

I don't know the exact language spec that makes this so, but since the assertion isn't dependent on a template parameter, the compiler can eagerly reject the code here. This will fail to compile with the v143 toolchain and later.
The solution is to simply make the expression depend on T, for instance using sizeof(T) == 0.

[StephenLPeters]

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[StephenLPeters]

@lhecker looks good to me!