Add experimental option to use OpenJDK 11 toolchain.

With this change, setting the environment variable
EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN=true switches from the OpenJDK 9
toolchain to the OpenJDK 11 one (prebuilts/jdk/jdk11).

Since the version of the java.base module has to match the version of
the jlink tool which consumes it, the --module-version argument to the
jmod create call is also switched.

Test: make
Test: EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN make
Bug: 131683177
Change-Id: I606d22538165f309fe5537bd67a26390b1dd5771
diff --git a/java/config/makevars.go b/java/config/makevars.go
index c40f4fc..2fa6f89 100644
--- a/java/config/makevars.go
+++ b/java/config/makevars.go
@@ -38,6 +38,7 @@
 	ctx.Strict("ANDROID_JAVA_HOME", "${JavaHome}")
 	ctx.Strict("ANDROID_JAVA8_HOME", "prebuilts/jdk/jdk8/${hostPrebuiltTag}")
 	ctx.Strict("ANDROID_JAVA9_HOME", "prebuilts/jdk/jdk9/${hostPrebuiltTag}")
+	ctx.Strict("ANDROID_JAVA11_HOME", "prebuilts/jdk/jdk11/${hostPrebuiltTag}")
 	ctx.Strict("ANDROID_JAVA_TOOLCHAIN", "${JavaToolchain}")
 	ctx.Strict("JAVA", "${JavaCmd} ${JavaVmFlags}")
 	ctx.Strict("JAVAC", "${JavacCmd} ${JavacVmFlags}")