fix(auth): mitigate tapjacking on the auth activity window

[just1and0]

Summary

• Adds window.decorView.filterTouchesWhenObscured = true in FirebaseAuthActivity.onCreate() to mitigate tapjacking/clickjacking attacks where an overlay intercepts user touches on the auth UI.
• FirebaseAuthActivity is the only activity in the auth module (v10 is Compose-based), so this single-line change covers the entire auth flow.

Fixes #2041

Test plan

• Launch the auth flow and verify touch input works normally without overlays
• Verify that touches are ignored when another app draws an overlay on top of the auth UI

[gemini-code-assist]

Code Review

This pull request updates FirebaseAuthActivity by setting window.decorView.filterTouchesWhenObscured to true in onCreate. This is a security improvement to prevent tapjacking by filtering touch events when the window is obscured. There are no review comments, and I have no feedback to provide.