Fixed platform detection for Android

Conflicts:
	Source/OpenTK/Configuration.cs
Fixed ES rendering.
Added SDL_GL_GetCurrentContext method and TouchMouseID constant.

SDL_GL_GetCurrentContext will allow us to replace the weird
implementation in Sdl2Factory.CreateGetCurrentGraphicsContext()
and the latter to disable mouse emulation if we wish to.
Conflicts:
	Source/OpenTK/Platform/SDL2/Sdl2.cs
More sane Sdl2Factory.CreateGetCurrentContext() implementation
Added missing doc comment