[Android] Apply C# 12 primary constructor to all MainApplication files

[melgoharyme]

Summary

• Updated all MainApplication.cs files under the Android folders across every project and sample in this repository.
• Adopted C# 12 primary constructor syntax for a cleaner and more consistent structure.
• Verified that all affected projects build successfully.
• Performed minor cleanups to maintain consistency across the codebase.

Notes

• No functional changes were introduced.
• This PR focuses on modernization and consistency only.