Remove obsolete AndroidLeakFixes after minimum SDK upgrade

Removes 5 AndroidLeakFixes enum constants that are no longer needed
with minimum SDK 26+:

- MEDIA_SESSION_LEGACY_HELPER (API 21 only)
- CONNECTIVITY_MANAGER (API ≤23 only)
- ACTIVITY_MANAGER (Samsung API 22 only)
- IMM_FOCUSED_VIEW (API ≤23 only)
- SPELL_CHECKER (API 23 only)

These fixes were already no-ops with early returns after the min SDK
upgrade. Removing them completely eliminates ~200 lines of dead code
and documentation while maintaining all functional leak fixes.

The remaining 10 AndroidLeakFixes enum constants continue to provide
useful leak fixes for Android 26+.

Co-Authored-By: Claude <noreply@anthropic.com>