refactor(android)!: restructure to standard Capacitor plugin layout

Restructure Android project from nested module layout to standard
Capacitor plugin structure following community conventions.

Structure Changes:
- Move plugin code from android/plugin/ to android/src/main/java/
- Move test app from android/app/ to test-apps/android-test-app/app/
- Remove nested android/plugin module structure
- Remove nested android/app test app structure

Build Infrastructure:
- Add Gradle wrapper files (gradlew, gradlew.bat, gradle/wrapper/)
- Transform android/build.gradle from root project to library module
- Update android/settings.gradle for standalone plugin builds
- Add android/gradle.properties with AndroidX configuration
- Add android/consumer-rules.pro for ProGuard rules

Configuration Updates:
- Add prepare script to package.json for automatic builds on npm install
- Update package.json version to 1.0.1
- Update android/build.gradle to properly resolve Capacitor dependencies
- Update test-apps/android-test-app/settings.gradle with correct paths
- Remove android/variables.gradle (hardcode values in build.gradle)

Documentation:
- Update BUILDING.md with new structure and build process
- Update INTEGRATION_GUIDE.md to reflect standard structure
- Update README.md to remove path fix warnings
- Add test-apps/BUILD_PROCESS.md documenting test app build flows

Test App Configuration:
- Fix android-test-app to correctly reference plugin and Capacitor
- Remove capacitor-cordova-android-plugins dependency (not needed)
- Update capacitor.settings.gradle path verification in fix script

BREAKING CHANGE: Plugin now uses standard Capacitor Android structure.
Consuming apps must update their capacitor.settings.gradle to reference
android/ instead of android/plugin/. This is automatically handled by
Capacitor CLI for apps using standard plugin installation.

test-apps/android-test-app/variables.gradle

@@ -0,0 +1,16 @@
+ext {
+    minSdkVersion = 23
+    compileSdkVersion = 35
+    targetSdkVersion = 35
+    androidxActivityVersion = '1.9.2'
+    androidxAppCompatVersion = '1.7.0'
+    androidxCoordinatorLayoutVersion = '1.2.0'
+    androidxCoreVersion = '1.15.0'
+    androidxFragmentVersion = '1.8.4'
+    coreSplashScreenVersion = '1.0.1'
+    androidxWebkitVersion = '1.12.1'
+    junitVersion = '4.13.2'
+    androidxJunitVersion = '1.2.1'
+    androidxEspressoCoreVersion = '3.6.1'
+    cordovaAndroidVersion = '10.1.1'
+}