libstagefright: Disable smoothstreaming for secure playback

Disable Smoothstreaming if the property "mm.disable.sec_smoothstreaming"
is set.

CRs-Fixed: 664732

Change-Id: I36da2e3bf73c802166e62ea3bfadeac523663b77
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