stagefright: Add support for loading a custom OMXPlugin

 * To facilitate moving the stagefright-plugins glue out of the
   framework, support is added to OMXMaster to load multiple
   external plugins besides internal/vendor versions.
 * This is currently limited to one plugin, defined by the
   "mm.sf.omx-plugin" system property. The code will allow any
   number of libraries to be loaded, though.
 * Should also be useful for nonstandard vendor implementations too.

Change-Id: I27d7e16ad56baa17754d8ea47a8c608a0d73d6f1