Don't generate dex jars for host modules

Hostdex support isn't working yet, and we are accidentally producing
a dex jar for host modules, which confuses soong_java_prebuilt.mk.

Test: m -j checkbuild
Change-Id: Ie2c331fa697f49dab23c380a5c6e0490d67c9f74
diff --git a/java/java.go b/java/java.go
index 326d278..43f97ac 100644
--- a/java/java.go
+++ b/java/java.go
@@ -466,7 +466,8 @@
 
 	j.classpathFile = outputFile
 
-	if j.deviceProperties.Dex && len(srcFiles) > 0 {
+	// TODO(ccross): handle hostdex
+	if ctx.Device() && len(srcFiles) > 0 {
 		dxFlags := j.deviceProperties.Dxflags
 		if false /* emma enabled */ {
 			// If you instrument class files that have local variable debug information in