Add explicit values to ABI enums in libs/hwui

In a separate code review we agreed that at ABI boundaries it feels nice
to explicitly call out enum values rather than rely on the rules of
C/C++ which others may not be comfortable with.

This CL adds explicit values to enums inside draw_gl.h.

Test: I built and tested on a Pixel 2.
Change-Id: I64c03e2684c1ab096a9c0665e4ed3d8b7bb22ac7