[Inflight regression] Fixed Test failures ModalTabbedPagePushAsyncShouldOverlayBottomNavigationView and GroupedCollectionViewScrollToIndexScrollsToCorrectItem

[Dhivya-SF4094]

This pull request improves both the scrolling logic for grouped data in Android's MauiRecyclerView and the reliability and clarity of UI tests for modal navigation with tabbed pages. The most important changes are grouped below:

Android RecyclerView scrolling improvements:

• Added a direct mapping from flat data indices to RecyclerView adapter positions for grouped data, improving performance and correctness when scrolling to items by flat index. This avoids ambiguity and unnecessary item comparisons when items in different groups are equal.
• Refactored the logic so that flat index requests no longer materialize the item for position lookup, further reducing overhead and potential errors.

UI test improvements for modal tabbed navigation:

• Reorganized and clarified the test methods for modal tabbed page navigation: the tests are now ordered, more clearly named, and include explicit steps to reset state between tests, increasing reliability and maintainability. (Issue35331.cs)
• Swapped the order and logic of the two test methods to better separate concerns: one test now verifies that returning from a detail page restores the tab bar, and the other confirms that the pushed detail page overlays the tab bar as expected. (Issue35331.cs)