Setup java paths in soong_ui

This way config.mk no longer needs to check which java is in PATH and
fix it. It'll be consistent for all build steps under soong_ui.

Also unify handling of ANDROID_JAVA_HOME / JAVA_HOME with
OVERRIDE_ANDROID_JAVA_HOME / EXPERIMENTAL_USE_OPENJDK9.

Test: m nothing
Test: build/soong/soong_ui.bash --make-mode nothing (w/o envsetup.sh)
Test: aosp_arm ninja files are the same before/after
Test: before/after ninja files match with OVERRIDE_ANDROID_JAVA_HOME
Test: before/after ninja files match with EXPERIMENTAL_USE_OPENJDK9
Change-Id: Icdb65093d9c346524074de239a4f895e4230a24d
diff --git a/java/config/config.go b/java/config/config.go
index 654d935..d4994a1 100644
--- a/java/config/config.go
+++ b/java/config/config.go
@@ -55,13 +55,8 @@
 	pctx.VariableConfigMethod("hostPrebuiltTag", android.Config.PrebuiltOS)
 
 	pctx.VariableFunc("JavaHome", func(config interface{}) (string, error) {
-		if override := config.(android.Config).Getenv("OVERRIDE_ANDROID_JAVA_HOME"); override != "" {
-			return override, nil
-		}
-		if config.(android.Config).UseOpenJDK9() {
-			return "prebuilts/jdk/jdk9/${hostPrebuiltTag}", nil
-		}
-		return "prebuilts/jdk/jdk8/${hostPrebuiltTag}", nil
+		// This is set up and guaranteed by soong_ui
+		return config.(android.Config).Getenv("ANDROID_JAVA_HOME"), nil
 	})
 
 	pctx.SourcePathVariable("JavaToolchain", "${JavaHome}/bin")