Cleanup header and build targets for libhwui clients.

Targets that will also be contained within the UI module
are allowed to access internal headers.  All other targets
that depend on libhwui are restricted to using the APEX headers.

Bug: 137655431
Test: CtsUiRenderingTestCases
Change-Id: Id92e9874dafb98bd79839d45ab8f22ab999689de

native/android/Android.bp

@@ -81,6 +81,8 @@ cc_library_shared {
         "libarect",
     ],
 
+    header_libs: [ "libhwui_internal_headers" ],
+
     whole_static_libs: ["libnativedisplay", "libnativewindow"],
 
     export_static_lib_headers: ["libarect"],