Add test for mp3 decoder

The test accepts a mp3 file as an input and
dumps output PCM in a wav file.

[x] Command to run the test
    libstagefright_mp3dec_test <input mp3> <output wav>

[x] Sources
    MP3 reader portion of test contains code from
    frameworks/av/media/libstagefright/MP3Extractor.cpp.

Bug: 17722566
Change-Id: If4586a7cd6d5bc9cecea34aacfb89db458ea3814
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>