Remove mojo/edk/embedder/entrypoints.h and {Get,Set}Core().

Add embedder_internal.h and expose g_core directly. (I want to do this,
since I want to add another such global (ChannelManager), and the
{Get,Set}Core() abstraction is overkill. Moreover, entrypoints.h wasn't
meant for consumption by users of the embedder API.)

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/721873002