Prevent apex from using preferred prebuilt_boot_image

Bug: 177892522
Test: m nothing
Change-Id: I8530ffa4c5123f6055a6ca25421c17bb6fc037f0
diff --git a/apex/apex.go b/apex/apex.go
index dca5595..66c598c 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -567,7 +567,7 @@
 	certificateTag  = dependencyTag{name: "certificate"}
 	executableTag   = dependencyTag{name: "executable", payload: true}
 	fsTag           = dependencyTag{name: "filesystem", payload: true}
-	bootImageTag    = dependencyTag{name: "bootImage", payload: true}
+	bootImageTag    = dependencyTag{name: "bootImage", payload: true, sourceOnly: true}
 	compatConfigTag = dependencyTag{name: "compatConfig", payload: true, sourceOnly: true}
 	javaLibTag      = dependencyTag{name: "javaLib", payload: true}
 	jniLibTag       = dependencyTag{name: "jniLib", payload: true}