[Android] Corrected GetCurrentContext()

EGLContext.Handle returns the handle of the JNI object, not the handle
of the native EGL context. This means that different ContextHandles are
returned for the same EGL context.

According to the EGLContext documentation, GetHashCode() has the
expected behavior - the same hash code is always returned for the same
EGL context.
[Android] Fixed extension loading and Context

OpenGL ES entry points are now loaded correctly according to the
Android / EGL documentation: static exports are loaded through dlsym,
whereas dynamic exports (extensions) are loaded through
eglGetProcAddress.

Additionally, the Context property has been updated to return
consistent results with AndroidFactory.CreateGetCurrentContext().
[Android] Fixed compilation issue

Upstream bug report: https://bugzilla.xamarin.com/show_bug.cgi?id=20030
[Android] Initialize AndroidFactory

When running on Android, we now initialize the correct AndroidFactory
IPlatformFactory instance.
[Android] Route GraphicsContext through facade

The AndroidGraphicsContext is now constructed through the
GraphicsContext facade. This ensures our internal housekeeping remains
consistent and fixes an issue where GraphicsContext.CurrentContext
returns null, causing extension loading to fail.

Additionally, this contains a fix for
https://bugzilla.xamarin.com/show_bug.cgi?id=20030
[ES] Align ES and GL RenderbufferStorage versions

Both versions of RenderbufferStorage now accept a RenderbufferStorage
enum as internal format. ES overloads where added to remain compatible
with the RenderbufferInternalFormat enum.
Regenerated bindings
[Android] Fix compilation issues

The resulting dll is now named OpenTK-1.1.dll to avoid issue
https://bugzilla.xamarin.com/show_bug.cgi?id=18575

The library now uses OPENTK_1_1 #define to distinguish itself from
previous versions. Avoids issue:
https://bugzilla.xamarin.com/show_bug.cgi?id=20030

Unix DL bindings are now included (dlopen, dlsym, etc…), in order to
support extension loading on Android.
[Examples] Update to OpenTK-1.1 bindings