libstagefright: Extended media support via FFMPEG

 * Original work by Michael Chen - https://github.com/omxcodec
 * Kitkat port from Android X86 project

libstagefright: add null checking to addPlugin

libstagefright: fix error handling of dlsym()

According 'man dlsym', a NULL return from dlsym() doesn't indicate an error.
The correct way to test for an error is to call dlerror() to clear any old
error conditions, then call dlsym(), and then call dlerror() again to check
whether its return value is not NULL.

libstagefright: add more media mimetypes

This is the first step to add the ffmpeg plugins.

libstagefright: add ExtractorPlugin

The patch allows to load an ExtractorPlugin to extend the functions of
DataSource::sniff and MediaExtractor. A plugin has to implement a C
function getExtractorPlugin to fill the MediaExtractor::Plugin struct.

The filename of the plugin could be specified by the
media.stagefright.extractor-plugin property.

Change-Id: I995a37a4f1ab4bba6ca3c24c7001a27a1e3ccb90

FLACExtractor: Add more sample rates support

In FLACExtractor.cpp, it has function to check file's sample rate.
If the input sample rate is not in its list, it will return unsupported