AMR-NB encoder: Add test

The test accepts a raw PCM file as input and generates
.amr file as output.

To run the test, use following command
libstagefright_amrnbenc_test [options] <input pcm file> <output amr file>

Bug: 17722566
Change-Id: I4429af482223a1b28e847f91349900706c468d75
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>