Remove direct dependency of external/skia on frameworks/native

Moves all code that needs to link with libgui or libui into
frameworks/native/hwui/utils/TestWindowContext. This allows us to
run Skia automated tests against a HWUI backend to watch for
performance or correctness regressions on Android.

BUG=23556017
TEST=After this change, we can remove libui and frameworks/native/include
from external/skia/Android.mk
R=djsollen@google.com

Change-Id: I6f02a5ff98101e448606a1e892686e2c648b6c8a