Support experimental building with OpenJDK 21

Use EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN=true to build with OpenJDK 21
while still targeting java language version 17.

Bug: 313924276
Test: m EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN=true
Change-Id: Idc892bb7519e597f1e280ca0765c1a281bb29955
diff --git a/ui/build/config.go b/ui/build/config.go
index d345415..613fc65 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -384,10 +384,14 @@
 	// Configure Java-related variables, including adding it to $PATH
 	java8Home := filepath.Join("prebuilts/jdk/jdk8", ret.HostPrebuiltTag())
 	java17Home := filepath.Join("prebuilts/jdk/jdk17", ret.HostPrebuiltTag())
+	java21Home := filepath.Join("prebuilts/jdk/jdk21", ret.HostPrebuiltTag())
 	javaHome := func() string {
 		if override, ok := ret.environ.Get("OVERRIDE_ANDROID_JAVA_HOME"); ok {
 			return override
 		}
+		if ret.environ.IsEnvTrue("EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN") {
+			return java21Home
+		}
 		if toolchain11, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN"); ok && toolchain11 != "true" {
 			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN is no longer supported. An OpenJDK 11 toolchain is now the global default.")
 		}