Why StagefrightPlayer and AwesomePlayers are available? why the application cant directly calling the AwesomePlayer instead of StagefrightPlayer and internally how Stagefrightplayer is calling AwesomePlayer ???

In OpenCore also, as similar to AwesomePlayer we will have PlayerDriver.
All things will be routed via PlayerDriver from PvPlayer class.

  AwesomePlayer is a proxy to stagefrightPlayer . It is using pimpl design pattern.This is to reduce the header file resources needed.
If we have to use AwesomePlayer directly we need to include more header files related with Awesomeplayer. This is covered in stagefright player.So the user can use stagefrightplayer.h header file without
adding multiple headers files in libmediaplayer/mediaplayer.cpp

 To know more about PIMPL design pattern:
http://www.codeproject.com/Articles/17536/Use-of-PIMPL-Design-Pattern