Change PathInternal.IsCaseSensitive to a constant (#54340)

Change PathInternal.IsCaseSensitive to a constant (#54340)

* Return constants in PathInternal.GetIsCaseSensitive() on mobile platforms

   In particular on Android probing using I/O is slow and contributes to slow app startup.

   Fixes https://github.com/dotnet/runtime/issues/54339

* Implement Path.IsCaseSensitive as PathInternal.IsCaseSensitive

   Also Path.StringComparison => PathInternal.StringComparison

* Add test for PathInternal.IsCaseSensitive

   Move GetIsCaseSensitiveByProbing to FileSystemTest

* Drop PathInternal.s_isCaseSensitive cache field

* Delete Path.IsCaseSensitive and Path.StringComparison

   update callers to use PathInternal.IsCaseSensitive and PathInternal.StringComparison

* Remove catch clause from GetIsCaseSensitiveByProbing

* Mark new test [OuterLoop]

* Apply suggestions from code review

Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
Co-authored-by: Adam Sitnik <adam.sitnik@gmail.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>