Support experimental building with OpenJDK 17

Use EXPERIMENTAL_USE_OPENJDK17_TOOLCHAIN=true to build with OpenJDK 17
while still targeting java language version 11.

Bug: 219098645
Test: m EXPERIMENTAL_USE_OPENJDK17_TOOLCHAIN=true
Change-Id: I89fb04ecde4dbd1e102098f316692624dd67f8a2

java/config/config.go

@@ -105,7 +105,12 @@ func init() {
 		if override := ctx.Config().Getenv("OVERRIDE_JLINK_VERSION_NUMBER"); override != "" {
 			return override
 		}
-		return "11"
+		switch ctx.Config().Getenv("EXPERIMENTAL_USE_OPENJDK17_TOOLCHAIN") {
+		case "true":
+			return "17"
+		default:
+			return "11"
+		}
 	})
 
 	pctx.SourcePathVariable("JavaToolchain", "${JavaHome}/bin")