Build: Experimental option to default to -source 1.9 -target 1.9

Enabled via:

export EXPERIMENTAL_USE_OPENJDK9=true

Other nonempty values of EXPERIMENTAL_USE_OPENJDK9 will
allow OpenJDK 9 toolchains, but will still default to
-source 1.8 and -target 1.8.

Note that -source 1.9 and -target 1.9 does not currently
successfully build.

Test: Treehugger.
Bug: 38177569

Experimental flag to set LOCAL_JAVA_LANGUAGE_VERSION := 1.9

Change-Id: I9eb881b3fbd1806984a132f6da7b5a4cc6612247
diff --git a/core/java_common.mk b/core/java_common.mk
index 3c9b18f..ba00b368 100644
--- a/core/java_common.mk
+++ b/core/java_common.mk
@@ -19,7 +19,11 @@
   ifneq (,$(filter $(LOCAL_SDK_VERSION), $(private_sdk_versions_without_any_java_18_support)))
     LOCAL_JAVA_LANGUAGE_VERSION := 1.7
   else
-    LOCAL_JAVA_LANGUAGE_VERSION := 1.8
+    ifneq ($(EXPERIMENTAL_USE_OPENJDK9),true)
+      LOCAL_JAVA_LANGUAGE_VERSION := 1.8
+    else
+      LOCAL_JAVA_LANGUAGE_VERSION := 1.9
+    endif
   endif
 endif
 LOCAL_JAVACFLAGS += -source $(LOCAL_JAVA_LANGUAGE_VERSION) -target $(LOCAL_JAVA_LANGUAGE_VERSION)