split hwcomposer.h

split hwcomposer.h

hwcomposer_defs.h now contains enums, which are essentially names.
This won't change when we rev the h/w composer API (new ones will be added instead).

we do this to avoid having to abstract all these constants in higher
level APIs; instead hwcomposer_defs.h can be included without exposing
the hwcomposer's data structures (which are a lot more fragile wrt.
backward compatibility)

Change-Id: Ifb514f64de02a599fdd2d31c188327209ccb0ffc