Provide a Rule for overriding app locale in instrumented tests

Please provide a standard way to override the app's Locale in tests. Main use cases for me are testing against a given language translation, and for tests on screens that display currency amount where the format depends on your locale, eg:
- en_US displays `GBP2.00`
- en_GB displays `£2.00`

so I need to set a locale to make sure my UI assertions match the locale-dependent text.

I currently use

```kotlin
class LocaleOverrideRule(
private val getLocale: () -> Locale,
) : ExternalResource() {
private lateinit var previousLocale: Locale
override fun before() {
previousLocale = Locale.getDefault()
val newLocale = getLocale()

Locale.setDefault(newLocale)
// We also have to update Locale in the Configuration, otherwise Locale.setDefault() gets
// overwritten with Configuration.locale's value at startup.
val resources: Resources = InstrumentationRegistry.getInstrumentation().targetContext.resources
val config: Configuration = resources.configuration
config.setLocale(newLocale)
resources.updateConfiguration(config, resources.displayMetrics)

println("Setting locale from $previousLocale to ${Locale.getDefault()}")
println("Format locale is now ${Locale.getDefault(Locale.Category.FORMAT)}")
}

override fun after() {
println("Restoring locale from ${Locale.getDefault()} to $previousLocale")
Locale.setDefault(previousLocale)
}
}
```

which works, but it relies on the deprecated `Resources.updateConfiguration()`. It recommends replacing that with `Context.createConfigurationContext(Configuration)` but I don't see an obvious way to provide a context that the ActivityScenario should use. The upcoming `AppCompatDelegate.setAppLocales()` may work for this purpose.

Either way, I feel like this is stuff is hard enough to work out that AndroidX test should provide a built-in solution.

The issue's LocaleOverrideRule example is the starting point; review how instrumented tests provide contexts to ActivityScenario and how locale configuration is applied. Done means AndroidX Test offers a documented, built-in locale override for translation and locale-dependent UI assertions without relying on deprecated Resources.updateConfiguration().

Written by the indexing model from the issue text.