Refactors the hardware acceleration mechanism by making AVCodecContext
a class member in HardwareAccel, wrapping extractData in a base
method to avoid duplicating boilerplate code, making the AccelID
enum and AccelInfo struct visible only to the makeHardwareAccel
function and adds more flexibility in the compilation of Ring's
hardware acceleration.

Change-Id: Ibfb4324e2f84b4d08bb5dd1e4b22873255aeb196