HWUI: reset buffer state after MakeCurrent

MakeCurrent call is not guaranteed to reset read/draw buffer state of
default framebuffer unless it's the first time the context is bound.
When the default framebuffer transitions from EGL_NO_SURFACE to a surface,
it's buffer state might stay GL_NONE.

This change make sure default framebuffer buffer state is GL_BACK after
MakeCurrent.

Change-Id: I9d8a402c6e66e8eca177d8dec8208db3abb155f2