Dedup code for generating prebuilt_stubs_sources in snapshot

Removes duplicated code for generating a versioned and non-versioned
form of the prebuilt_stubs_sources module in the snapshot.

Bug: 143678475
Test: m nothing

Change-Id: Ic431b6fcb6f3c7c85f1a04cb6f087424aaf0cffa

sdk/sdk_test.go

@@ -515,8 +515,8 @@ prebuilt_stubs_sources {
 
 prebuilt_stubs_sources {
     name: "myjavaapistubs",
-    srcs: ["java/myjavaapistubs_stubs_sources"],
     prefer: false,
+    srcs: ["java/myjavaapistubs_stubs_sources"],
 }
 
 cc_prebuilt_library_shared {