1) Add the include path for jni.h.
2) Rename InitCheck() and read_at() to initCheck() and readAt(),
repectively in StreamSource, as done in DataSource.
3) Add back the definition of SkJPEGImageDecoder to jpeg_decoder_bench.cpp.
Need to do so because SkJPEGImageDecoder is not defined in a header file.
Add StreamSource, which wraps a SkStream to a DataSource.
Add SkOmxPixelRef, which holds a MediaBuffer.
Test OmxJpegImageDecoder by a Jpeg decoder bench.
