Add overridable attachContent() to allow non-RIB roots in RibActivity

[dipanshu281]

Summary

Currently there isn't a way to test RIB vs non-RIB architecture without creating two different activities to bypass RIB creation code in RibActivity. Two activities introduce additional latency on startup path. This PR decouples the RIB creation block from onCreate() so it can be no-oped in one of the branches for a test.

Change

• Extract the root-attach block from onCreate into a new protected open fun attachContent(rootViewGroup, savedInstanceState). The default implementation reproduces the current inline logic, so behavior for existing subclasses is unchanged.
• Drop the ?: throw NullPointerException("Router should not be null") in onSaveInstanceState. Because router is already declared nullable, the elvis-safe delegation is semantically identical when router != null (the common case); the null case now no-ops cleanly instead of NPE-ing when a subclass overrides attachContent to skip attach.
• onDestroy and onBackPressed were already null-safe; no change.

Backward compatibility

• API: additive. attachContent is a new protected open method with a default that reproduces the current inline logic.
• Behavior: no change for any subclass that inherits the default attachContent.
• onSaveInstanceState: identical behavior when router != null; the previously-guaranteed-then-thrown null path becomes a no-op.

Tests

Added RibActivityTest cases exercising a subclass that overrides attachContent to skip attach:

• onCreate_whenAttachContentOverriddenToSkip_shouldLeaveRouterNull — attachContent runs, createRouter is not invoked, and interactor access throws IllegalStateException.
• onCreate_whenAttachContentOverriddenToSkip_shouldStillPublishLifecycle — lifecycle() still emits CREATE.
• onBackPressed_whenRouterIsNull_shouldFallThroughToSuper — onUnhandledBackPressed() runs and the activity finishes.
• onSaveInstanceState_whenRouterIsNull_shouldNotCrash — no NPE.

Full :libraries:rib-android:test suite passes locally (17/17 in RibActivityTest).

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