Fix dump-oat-boot build target.

Implement support of dumping host boot.art.

That was not possible before because imageLocation is different on host.

This CL makes imageLocation per-variant so can access for the host variant.

Test: m dump-oat-boot
Change-Id: Id82161e4901a6b77f220238aec460b8609fd765f
diff --git a/java/dexpreopt.go b/java/dexpreopt.go
index 40cfe4f..bf64ae8 100644
--- a/java/dexpreopt.go
+++ b/java/dexpreopt.go
@@ -155,6 +155,8 @@
 		images = append(images, variant.images)
 		imagesDeps = append(imagesDeps, variant.imagesDeps)
 	}
+	// The locations for all Android targets are identical. Pick the first one.
+	imageLocations := bootImage.getVariant(targets[0]).imageLocations()
 
 	dexLocation := android.InstallPathToOnDevicePath(ctx, d.installPath)
 
@@ -198,7 +200,7 @@
 		Archs:                   archs,
 		DexPreoptImages:         images,
 		DexPreoptImagesDeps:     imagesDeps,
-		DexPreoptImageLocations: bootImage.imageLocations,
+		DexPreoptImageLocations: imageLocations,
 
 		PreoptBootClassPathDexFiles:     dexFiles,
 		PreoptBootClassPathDexLocations: dexLocations,