Fix wrong int argument of FOURCC.

Clang compiler reports error for aosp_x86-eng target:

.../MPEG4Extractor.cpp:1538:14: error: case value is not a constant expression
case FOURCC('\xA9', 'x', 'y', 'z'):
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
..../MPEG4Extractor.cpp:1538:14: note: left shift of negative value -87

Change-Id: I3ef1aa710cd1ce84ca46cdfbe37876bb9a1140bd