Remove import_includes/export_includes

Now that ONE_SHOT_MAKEFILE no longer exists, we don't have to rely on
the filesystem to store this informtion.

This removes ~16.7k files from our build graph
(aosp-master/aosp_arm64-eng), though only about 600 of them were being
used in a normal build.

Test: treehugger
Change-Id: I3ac12f5ea7f11d25064109a0599bc5be1976fba5
diff --git a/core/definitions.mk b/core/definitions.mk
index 16bec84..f32a995 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -1107,7 +1107,7 @@
 ###########################################################
 define c-includes
 $(addprefix -I , $(PRIVATE_C_INCLUDES)) \
-$$(cat $(PRIVATE_IMPORT_INCLUDES))\
+$(foreach i,$(PRIVATE_IMPORTED_INCLUDES),$(EXPORTS.$(i)))\
 $(if $(PRIVATE_NO_DEFAULT_COMPILER_FLAGS),,\
     $(addprefix -I ,\
         $(filter-out $(PRIVATE_C_INCLUDES), \