[droid] enable gnutls support in ffmpeg

this bumps gnutls to last stable v 3.3.10 and enables gnutls support in ffmpeg on android (again).

I can't remember the exact reason why it was disabled for gotham - some unresolved symbols or the locking callbacks maybe - but it should be fixed with moving ffmpeg to depends.

I've been running this on ADT-1 for a while and didn't notice issues related to it.
It also got some user testing, see http://forum.kodi.tv/showthread.php?tid=209569