Make ANDROID_JAVA{8,9}_HOME available to config.mk.

Previously, these variables were exported only by makevars.go,
but those values are not available to config.mk. This CL adds
the variable to ui/build/config.go, which also makes it
available to config.mk.

Test: Treehugger
Bug: 70862583
Bug: 70521453
Change-Id: Ib54660e4b08ab751265b30004630cf1bb8c0041b
diff --git a/ui/build/config.go b/ui/build/config.go
index df97d80..c975243 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -146,14 +146,16 @@
 	}
 
 	// Configure Java-related variables, including adding it to $PATH
+	java8Home := filepath.Join("prebuilts/jdk/jdk8", ret.HostPrebuiltTag())
+	java9Home := filepath.Join("prebuilts/jdk/jdk9", ret.HostPrebuiltTag())
 	javaHome := func() string {
 		if override, ok := ret.environ.Get("OVERRIDE_ANDROID_JAVA_HOME"); ok {
 			return override
 		}
 		if v, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK9"); ok && v != "" && v != "false" {
-			return filepath.Join("prebuilts/jdk/jdk9", ret.HostPrebuiltTag())
+			return java9Home
 		}
-		return filepath.Join("prebuilts/jdk/jdk8", ret.HostPrebuiltTag())
+		return java8Home
 	}()
 	absJavaHome := absPath(ctx, javaHome)
 
@@ -164,6 +166,8 @@
 	ret.environ.Unset("OVERRIDE_ANDROID_JAVA_HOME")
 	ret.environ.Set("JAVA_HOME", absJavaHome)
 	ret.environ.Set("ANDROID_JAVA_HOME", javaHome)
+	ret.environ.Set("ANDROID_JAVA8_HOME", java8Home)
+	ret.environ.Set("ANDROID_JAVA9_HOME", java9Home)
 	ret.environ.Set("PATH", strings.Join(newPath, string(filepath.ListSeparator)))
 
 	return Config{ret}