Distinguish boot images by target rather than arch

We plan to add boot image variants for host tests.
Distinguishing the variants by arch does not work,
since both host and device can have the same arch.

Change-Id: Iea73c77367affb074f97a0fc318389417ce537da
diff --git a/java/dexpreopt.go b/java/dexpreopt.go
index 4313964..40cfe4f 100644
--- a/java/dexpreopt.go
+++ b/java/dexpreopt.go
@@ -132,28 +132,28 @@
 		bootImage = artBootImageConfig(ctx)
 	}
 
-	var archs []android.ArchType
-	for _, a := range ctx.MultiTargets() {
-		archs = append(archs, a.Arch.ArchType)
-	}
-	if len(archs) == 0 {
+	targets := ctx.MultiTargets()
+	if len(targets) == 0 {
 		// assume this is a java library, dexpreopt for all arches for now
 		for _, target := range ctx.Config().Targets[android.Android] {
 			if target.NativeBridge == android.NativeBridgeDisabled {
-				archs = append(archs, target.Arch.ArchType)
+				targets = append(targets, target)
 			}
 		}
 		if inList(ctx.ModuleName(), global.SystemServerJars) && !d.isSDKLibrary {
 			// If the module is not an SDK library and it's a system server jar, only preopt the primary arch.
-			archs = archs[:1]
+			targets = targets[:1]
 		}
 	}
 
+	var archs []android.ArchType
 	var images android.Paths
 	var imagesDeps []android.OutputPaths
-	for _, arch := range archs {
-		images = append(images, bootImage.images[arch])
-		imagesDeps = append(imagesDeps, bootImage.imagesDeps[arch])
+	for _, target := range targets {
+		archs = append(archs, target.Arch.ArchType)
+		variant := bootImage.getVariant(target)
+		images = append(images, variant.images)
+		imagesDeps = append(imagesDeps, variant.imagesDeps)
 	}
 
 	dexLocation := android.InstallPathToOnDevicePath(ctx, d.installPath)