android: change ANDROID_STL='c++_shared'

- fixes crashes in face-detector application
- libc++_shared.so is bundled automatically via CMake helper project

modules/java/android_sdk/libcxx_helper/CMakeLists.txt

@@ -0,0 +1,4 @@
+cmake_minimum_required(VERSION 3.6)
+
+# dummy target to bring libc++_shared.so into packages
+add_library(opencv_jni_shared STATIC dummy.cpp)

modules/java/android_sdk/libcxx_helper/dummy.cpp

@@ -0,0 +1 @@
+// empty