libstagefright: Add support to plug-in proprietary parser

 - Introduce an Extended Extractor to facilitate plugging-in
   of proprietary parsers.

 - QC Proprietary parsers will be built and added only if
   TARGET_ENABLE_QC_AV_ENHANCEMENTS is true.

Change-Id: I2aae9515867f0beaa822e17fa390a1dc81953965