Refactor project and update dependencies

- Update Gradle version to 8.7
- Update Android Gradle Plugin to 8.6.0
- Update compileSdkVersion to 36
- Update targetSdkVersion to 36
- Update minSdkVersion to 21 for `android-pdf-viewer` and 26 for `sample`
- Migrate to AndroidX
- Add Kotlin support
- Remove Android Annotations
- Update PdfiumAndroid to 1.9.4
- Add maven-publish script
- Configure JitPack build with JDK 17
- Update `settings.gradle` with pluginManagement and dependencyResolutionManagement
- Add `gradle.properties` for project-wide settings
- Add `android-pdf-viewer/gradle.properties` for artifact ID
- Update `sample` app to use PDFViewActivity directly
- Update dependencies in `sample/build.gradle`
- Update imports in source files
- Set `exported=true` for PDFViewActivity in `sample/src/main/AndroidManifest.xml`

Remove Bintray configuration and update Gradle

- Delete `bintray.gradle`
- Remove Bintray plugin from `build.gradle`
- Remove repository declarations from `sample/build.gradle`
- Update Gradle wrapper to version 8.6
- Update Android Gradle Plugin to 8.4.0
- Remove `ext` block from `android-pdf-viewer/build.gradle`
- Update `versionName` to "3.2.1" in `android-pdf-viewer/build.gradle`
- Change `com.android.support:support-compat` to `androidx.appcompat:appcompat` and make it `compileOnly`
- Remove `apply from: 'bintray.gradle'`

change pdfiumandroid version