java_library support for building headers-only

Flag for java_library modules to build just the Turbine headers and
skip building an impl jar.

Test: go test java
Bug: 289776578
Change-Id: Iad0babf951710476bc32df93c25d17065a14ab84
diff --git a/java/java.go b/java/java.go
index 70aba8e..c60226d 100644
--- a/java/java.go
+++ b/java/java.go
@@ -692,6 +692,7 @@
 	}
 
 	j.checkSdkVersions(ctx)
+	j.checkHeadersOnly(ctx)
 	if ctx.Device() {
 		j.dexpreopter.installPath = j.dexpreopter.getInstallPath(
 			ctx, android.PathForModuleInstall(ctx, "framework", j.Stem()+".jar"))