- eliminate CMake 3.5+ warnings
- removed usage of dropped "OPENCV_CAMERA_MODULES" variable
- partial support for Android NDK 11+

https://github.com/opencv/opencv/issues/7590

now it builds by the command:
`cmake.exe -Wno-dev -GNinja -DCMAKE_MAKE_PROGRAM="path\to\ninja\ninja.exe" -DCMAKE_TOOLCHAIN_FILE=../opencv3/platforms/android/android.toolchain.cmake -DANDROID_ABI="armeabi-v7a with NEON" -DANDROID_SDK_TARGET=21 -DANDROID_NATIVE_API_LEVEL=14 -DCMAKE_BUILD_WITH_INSTALL_RPATH=ON -DBUILD_ANDROID_EXAMPLES=ON -DINSTALL_ANDROID_EXAMPLES=ON -DWITH_OPENCL=YES -DANDROID_OPENCL_SDK=path\to\OpenCL ../opencv`

OPENCV_ANDROID_SDK - OpenCV Android SDK location (or OpenCV build dir)
Can be passed via ndk-build parameter or environment variable.

Camera and Camera2 preview is rendered via OpenGL textures and can be modified on CPU via C++ code. No OpenCL yet.

Add depends from highgui to imgcodecs and videoio. And include header from imgcodecs and videoio to imgcodecs

put the detection-based tracker back (to objdetect module), so that the Android face detection sample builds fine. The patch by Konstantin Matskevich that adds C++11 threads support has been merged in too. Also, fixed compile error in the doc builder.