stagefright amrnb: Properly negate all values

Initially, input values -32768 weren't negated properly.

This was committed on the master branch of the opencore
repository in commit f532d145194e474cb30d8644b8eee83873ad24cc,
based on AOSP contribution 10906.

Change-Id: I903353c5539a8fe94b7fc5f26b95eaeac57df5f4