stagefright: Add flag for enabling enhanced offload support

 * Some devices might not use the QCOM_HARDWARE flag, but still
   want to get the improved audio offload support.
 * Add TARGET_ENABLE_OFFLOAD_ENHANCEMENTS to enable support for
   PCM offload and 24-bit playback with no extra dependencies.

Change-Id: Ie1a92602c6c0cd59008db8dbf72125eecef08e85