Lifetime of activation arguments is too short and unclear

Describe the bug

The activation arguments have a short lifetime, which makes it easy to hit errors when trying to use them. The main issue is that this includes the data in the arguments, so one has to know to copy it out instead of just keeping a reference to it.

The documentation for AppInstance.GetActivatedEventArgs mentions that

for packaged apps, this method will only return the arguments the first time it is called in an app. So, you should call it as early as possible during activation, as the arguments may become unavailable later.

but it does not mention that even if one calls it early and stores the value for later, it may also become unavailable.

For file activation, the arguments' lifetime is tied to the caller process, so GetActivatedEventArgs may not even work the first time. See #5481

For activation redirection, the activation arguments are only valid during the Activated event. So, again, it is not possible to store the arguments and use them later.

Steps to reproduce the bug

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Expected behavior

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Screenshots

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NuGet package version

None

Packaging type

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Windows version

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IDE

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Additional context

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Start with the AppInstance.GetActivatedEventArgs API and the activation-redirection and file-activation behavior described in the issue; review related issue #5481. Determine the documented lifetime for each activation path and whether the reported unavailability is expected or requires a fix, then verify the resulting behavior or documentation.