Add test for AMR-NB 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_amrnbdec_test <input amr> <output wav>

Change-Id: I2492c2ed4e670c9fd8cb4213201ccbd0cb9bf8fe
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>