details: https://github.com/opencv/opencv/pull/13726

In order to help CMake and PkgConfig find FFmpeg you need to:
1) export `PKG_CONFIG_LIBDIR=${SYSROOT}`
where `SYSROOT=${ANDROID_NDK}/sysroot`
2) Add `-DCMAKE_PREFIX_PATH=${FFMPEG_INSTALL_PREFIX}`
3) Append `-DCMAKE_FIND_ROOT_PATH=${FFMPEG_INSTALL_PREFIX}`

* re-enabled NEON routines for arm64

Merge pull request #13803 from thangktran:thangktran/fix-misleading-runtime-assertion-calibrateCamera

objectPoints and imagePoints are not checked whether they're empty and
cause checkVector() to fail, thus result in a wrong error message.

Fixes: https://github.com/opencv/opencv/issues/6002

backport 997b7b18