re-introduce Cocos2dxHelper.runOnGLThread(Runnable)

It's crucial to execute various tasks synchonized with the rendering loop /
thread, but this ability (runOnGLThread) got removed with the NativeActivity
transitions a few days ago. This patch adds this method back again.

Why can't we just use runOnUiThread()? Because it seems not to be synchronized
properly and can cause broken OpenGL textures or even the whole app to crash.

Isn't there a better way to get into the mainloop? I hope so! But I'm not a
experienced Android developer and this solution is quite simple / the only one
that worked without any problems.
Merge pull request #3767 from michaelcontento/android-add-runOnGLThread-back-again

[ci skip]re-introduce Cocos2dxHelper.runOnGLThread(Runnable)