Make aGrep compatible with modern Androids

A ton of fixes, mostly AI-generated.

What's Changed

• Migrate project to AndroidX and Gradle app module by @kappa in #2
• Use FileProvider URIs for external text viewing by @kappa in #4
• Adopt SAF directory selection and update search permissions by @kappa in #3
• Refactor search to ViewModel architecture with ExecutorService by @kappa in #7
• Adopt Material settings UI and modern preference handling by @kappa in #6
• Replace native universalchardet with pure-Java juniversalchardet by @kappa in #8
• ...and more of that kind. But no functional changes.

Full Changelog: https://github.com/kappa/aGrep/commits/v1.0.0