AWindow: check if surfacetexture already exists

The SurfaceTextureListener attached to the TextureView won't be
signalled if the SurfaceTexture already exists and is already bound to
the TextureView. It means that a call to attachTextureView() will never
end to the expected call to setSurface(), blocking the playback in this
situation.

Pointed-out-by: Aleksandra Krzemień

Fixes #1587