Update HWUI makefiles to support use by other targets (e.g. Skia)

(1) Update both static and shared lib to export libhwui headers
(2) Work around problem in current make system where a static
    libraries dependencies are not transitively included by moving
    those dependencies into a file that can be included by targets
    using libhwui_static.
(3) Do not expose protobuf headers to clients outside of HWUI

Change-Id: I08470c02a83ae64116081eff0c2050878990c6ed