mediacodec: create SurfaceTexture in decoder

Initially, the SurfaceTexture reference was created through
AWindowHandler and typically was obtained through a TextureView. By
creating the SurfaceTexture ourselves, we can avoid requesting Android
libVLC user to use libvlc_media_player_set_android_context even if the
user doesn't need any display.

In addition, it means that we don't rely on code that is in the JNI
android binding instead of vlc core when using MediaCodec, which helps
other bindings like C# one to provide support for Android.

Signed-off-by: Alexandre Janniaux <ajanni@videolabs.io>