Solves issue #3925

    - [android] Added CV_CAP_PROP_OUTPUT_FORMAT to indicate desired output format for CvCapture_Android
    - [android] Added YUV420sp output format for CvCapture_Android

Solves the following: http://code.opencv.org/issues/3925.

Does not break existing functionality, as long as the `int flag` is not used in `VideoCapture::retrieve(OutputArray image, int flag)` for indicating the output format. This is the main thing this PR aims to change: Instead of this, e.g `VideoCapture::set(CV_CAP_PROP_OUTPUT_FORMAT, CV_CAP_PROP_ANDROID_COLOR_FRAME_BGRA)` should be set before reading the frame.