I have a platform-agnostic core engine that is extended by platform-specific 'back-ends' for Android and LWJGL.
Why do you need to do that? Doesn't LWJGL support Android??
No, or at least not yet AFAIK.
LWJGL nicely wraps up the underlying OpenGL APIs (and provides some other stuff such as display and mouse/keyboard device management).
Android has this stuff as part of it's core API (depending on version ofc) so they're sort of mutually-exclusive.
I believe most (all?) other libraries/engines use the same approach: LWJGL for Linux / Windows / Mac and OpenGL-ES for Android.