Test and fix bug checking status of secondary multidex files.

We were accidentally returning 'false' instead of 'true' to indicate a native
multidex file with an up-to-date primary dex file checksum but out-of-date
secondary dex file checksum is out of date.

Bug: 21022815
(cherry picked from commit a5a1c13a74225a1db3c54dc108fcd161b867bb00)

Change-Id: I0cc56d2d49858a4af91ca8b05aa8bd3c17bb44be
diff --git a/test/MultiDexModifiedSecondary/README.txt b/test/MultiDexModifiedSecondary/README.txt
new file mode 100644
index 0000000..4cf3a56
--- /dev/null
+++ b/test/MultiDexModifiedSecondary/README.txt
@@ -0,0 +1,4 @@
+MultiDexModifiedSecondary is designed to result in a multidex file that has
+the same classes.dex file as MultiDex, but a different classes2.dex.
+
+This is used in the OatFileAssistantTest.MultiDexSecondaryOutOfDate gtest.