MPEG4Source: fix fragmented read. Test: passed CTS test DecoderTest#testDecodeFragmented Bug: 64314728 Bug: 36571704 Change-Id: I71ad6aaae473b03483f8405899d3178148597bba (cherry picked from commit ba9af7792dfed6e9b1b216aab91a97e713eec891) (cherry picked from commit 6b401a337674f2f22b7589534700a33187899869)