Factor out a helper in SplitZipStep.

Summary:
I'm going to be adding some complexity to
createRequiredInPrimaryZipPredicate, so pull part of it out as a helper
function to make some room.

Test Plan: Unit.
diff --git a/src/com/facebook/buck/android/SplitZipStep.java b/src/com/facebook/buck/android/SplitZipStep.java
index 18196b3..ab468d4 100644
--- a/src/com/facebook/buck/android/SplitZipStep.java
+++ b/src/com/facebook/buck/android/SplitZipStep.java
@@ -168,36 +168,7 @@
   @VisibleForTesting
   Predicate<String> createRequiredInPrimaryZipPredicate(ExecutionContext context)
       throws IOException {
-    final ImmutableSet<String> classNames;
-    if (primaryDexClassesFile.isPresent()) {
-      Path manifest = primaryDexClassesFile.get();
-      classNames = FluentIterable
-          .from(context.getProjectFilesystem().readLines(manifest))
-          .transform(new Function<String, String>() {
-            @Override
-            public String apply(String line) {
-              // Blank lines should be ignored.
-              return line.trim();
-            }
-          })
-          .filter(new Predicate<String>() {
-            @Override
-            public boolean apply(String line) {
-              // Allow users to use # to comment out a line.
-              return !line.isEmpty() && !(line.charAt(0) == '#');
-            }
-          })
-          .transform(new Function<String, String>() {
-            @Override
-            public String apply(String line) {
-              // Append a ".class" so the input file can just contain the internal class name.
-              return line + ".class";
-            }
-          })
-          .toSet();
-    } else {
-      classNames = ImmutableSet.of();
-    }
+    final ImmutableSet<String> primaryDexClassNames = getPrimaryDexClassNames(context);
 
     return new Predicate<String>() {
       @Override
@@ -210,7 +181,7 @@
           return true;
         }
 
-        if (classNames.contains(name)) {
+        if (primaryDexClassNames.contains(name)) {
           return true;
         }
 
@@ -224,6 +195,39 @@
     };
   }
 
+  private ImmutableSet<String> getPrimaryDexClassNames(ExecutionContext context)
+      throws IOException {
+    if (!primaryDexClassesFile.isPresent()) {
+      return ImmutableSet.of();
+    }
+
+    Path manifest = primaryDexClassesFile.get();
+    return FluentIterable
+        .from(context.getProjectFilesystem().readLines(manifest))
+        .transform(new Function<String, String>() {
+          @Override
+          public String apply(String line) {
+            // Blank lines should be ignored.
+            return line.trim();
+          }
+        })
+        .filter(new Predicate<String>() {
+          @Override
+          public boolean apply(String line) {
+            // Allow users to use # to comment out a line.
+            return !line.isEmpty() && !(line.charAt(0) == '#');
+          }
+        })
+        .transform(new Function<String, String>() {
+          @Override
+          public String apply(String line) {
+            // Append a ".class" so the input file can just contain the internal class name.
+            return line + ".class";
+          }
+        })
+        .toSet();
+  }
+
   @VisibleForTesting
   static void writeMetaList(
       BufferedWriter writer,