fix: resolve Android build issues and create proper plugin module

- Fix capacitor.build.gradle to comment out missing Capacitor integration file
- Create separate plugin module with proper build.gradle configuration
- Copy plugin source code to android/plugin/src/main/java/
- Update settings.gradle to include plugin module
- Fix plugin build.gradle to remove Kotlin plugin dependency
- Successfully build plugin AAR: android/plugin/build/outputs/aar/plugin-release.aar
- Update BUILDING.md with correct build commands and troubleshooting

This resolves the Android Studio build issues by creating a proper
plugin library module separate from the test app.

android/app/capacitor.build.gradle

@@ -7,7 +7,8 @@ android {
   }
 }
 
-apply from: "../capacitor-cordova-android-plugins/cordova.variables.gradle"
+// Plugin development project - no Capacitor integration files needed
+// apply from: "../capacitor-cordova-android-plugins/cordova.variables.gradle"
 dependencies {