Add test for AMR-WB decoder

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

To run the test, use following command
libstagefright_amrwbdec_test <input amr> <output wav>

Bug: 17722566
Change-Id: Icbc780561b774cf02b48a6a23fd7c04e8e984ab6
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>