refactored; added Camera2, notify callbacks, front/back maxCamera sizes; disable new stuff if target API < 21

a simple sample will look like:

```java
public class MainActivity extends Activity implements CameraGLSurfaceView.CameraTextureListener {

	CameraGLSurfaceView mView;
	ByteBuffer buf;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON,
                WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);

        mView = new CameraGLSurfaceView(this, null);
        mView.setCameraTextureListener(this);
        setContentView(mView);
        buf = ByteBuffer.allocateDirect(1920*1080*4);
    }

    @Override
    protected void onPause() {
        mView.onPause();
        super.onPause();
    }

    @Override
    protected void onResume() {
        super.onResume();
        mView.onResume();
    }

	@Override
	public void onCameraViewStarted(int width, int height) {
		// TODO Auto-generated method stub

	}

	@Override
	public void onCameraViewStopped() {
		// TODO Auto-generated method stub

	}

	@Override
	public boolean onCameraFrame(int texIn, int texOut, int width, int height) {
		Log.i("MAIN", "onCameraFrame");
		int w=width, h=height;
		/*
		// option 1:
		// just return 'false' to display texIn on screen
		retutn false;
		*/

		/*
		// option 2:
		// fast copy texIn to texOut
		GLES20.glActiveTexture(GLES20.GL_TEXTURE0);
    	GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, texOut);
		GLES20.glCopyTexImage2D(GLES20.GL_TEXTURE_2D, 0, GLES20.GL_RGBA, 0, 0, w, h, 0);
		return true;
		*/

		// option 3:
		// read, modify and write back pixels
		GLES20.glReadPixels(0, 0, w, h, GLES20.GL_RGBA, GLES20.GL_UNSIGNED_BYTE, buf);

		buf.rewind();
		// red line
		for(int i=0; i<h; i++) {
			buf.position(w*4*i+i*4);
			buf.put((byte) -1);
			buf.position(w*4*i+i*4+4);
			buf.put((byte) -1);
		}
		buf.rewind();

		GLES20.glActiveTexture(GLES20.GL_TEXTURE0);
    	GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, texOut);
		GLES20.glTexSubImage2D(GLES20.GL_TEXTURE_2D, 0, 0, 0, w, h, GLES20.GL_RGBA, GLES20.GL_UNSIGNED_BYTE, buf);
		return true;
	}
}
```

Fix bug when enter 4 channel image to LineAA function.

https://github.com/Itseez/opencv/issues/4791