AAR package build and test with CI

- Use the same tools and plugins for SDK build and AAR build
- Added script to test Gradle-based samples against local maven repo
- Various local fixes and debug prints

samples/android/tutorial-2-mixedprocessing/build.gradle.in

@@ -53,8 +53,10 @@ android {
 dependencies {
     //implementation fileTree(dir: 'libs', include: ['*.jar'])
     if (gradle.opencv_source == "sdk_path") {
+        println 'Using OpenCV from SDK'
         implementation project(':opencv')
     } else if (gradle.opencv_source == "maven_local" || gradle.opencv_source == "maven_cenral") {
+        println 'Using OpenCV from Maven repo'
         implementation 'org.opencv:opencv:@OPENCV_VERSION_PLAIN@'
     }
 }