stagefright: Extra checks for HW AAC decoder

 * Don't engage the HW AAC codec if we don't have channel metadata.
 * This is the case for secure decode, in which case the HW codec
   performs badly.

Change-Id: I993fd01d3074eddc3ed9e867cf213d2da36f8d27
(cherry picked from commit d8695907e6132f04148d6ee141aa727e271e8fa9)