Use system modules generated by soong when using javac -target 1.9

Soong has support for building system modules.  Use the directories
produced by Soong with --system to replace -bootclasspath arguments
when using javac -target 1.9.

Since soong can't generate current SDK stubs yet, and no existing
SDK stubs need -target, only use -target 1.9 for modules that are
not compiling against the SDK.  That means in practice the only
system modules that will be used for now is the default one,
core-system-modules.

Bug: 63986449
Test: m -j EXPERIMENTAL_USE_OPENJDK9=true makes some progress
Change-Id: I350ef50aedf36fdd72458c23d4fe8a2edf1a9a02
diff --git a/core/definitions.mk b/core/definitions.mk
index 1f9891e..0e7093e 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -2251,9 +2251,11 @@
 $(hide) if [ -s $(PRIVATE_JAVA_SOURCE_LIST) ] ; then \
     $(SOONG_JAVAC_WRAPPER) $(1) -encoding UTF-8 \
     $(if $(findstring true,$(PRIVATE_WARNINGS_ENABLE)),$(xlint_unchecked),) \
-    $(addprefix -bootclasspath ,$(strip \
-        $(call normalize-path-list,$(PRIVATE_BOOTCLASSPATH)) \
-        $(PRIVATE_EMPTY_BOOTCLASSPATH))) \
+    $(if $(PRIVATE_USE_SYSTEM_MODULES), \
+      $(addprefix --system=,$(PRIVATE_SYSTEM_MODULES)), \
+      $(addprefix -bootclasspath ,$(strip \
+          $(call normalize-path-list,$(PRIVATE_BOOTCLASSPATH)) \
+          $(PRIVATE_EMPTY_BOOTCLASSPATH)))) \
     $(addprefix -classpath ,$(strip \
         $(call normalize-path-list,$(2)))) \
     $(if $(findstring true,$(PRIVATE_WARNINGS_ENABLE)),$(xlint_unchecked),) \
@@ -2300,6 +2302,7 @@
     $(3) \
     $(5) \
     $$(full_java_bootclasspath_libs) \
+    $$(full_java_system_modules_deps) \
     $$(layers_file) \
     $$(annotation_processor_deps) \
     $$(NORMALIZE_PATH) \