Fix the memory leak in the sample app

[NightXlt]

Replace the activity context to application context to prevent memory leak in Activity reconstruction case.

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[gemini-code-assist]

Code Review

This pull request updates the JsonProvider class to store the application context in a property. A review comment suggests marking this new context property as private to maintain encapsulation, as it is currently public by default.

[gemini-code-assist]

The context property is now public, which exposes the internal state of the class. It should be marked as private to maintain encapsulation, as it was in the previous version.

Suggested change

	val context = context.applicationContext
	private val context = context.applicationContext

[niharika2810]

HI Thanks for raising this. Can you check the sample if it is reproducible in that point your changes here then : https://github.com/android/identity-samples/tree/main/Shrine

as we might be deprecating or shifting compose sample to main soon.