manifest: switch back to r12 else ims will be broken

Change-Id: I6ab6de08dc5f91a6f6ab55458a706487f28eea8c

default.xml

@@ -15,7 +15,7 @@
            review="gerrit.omnirom.org"
            fetch="https://github.com/omnirom" />
 
-  <default revision="refs/tags/android-5.1.1_r13"
+  <default revision="refs/tags/android-5.1.1_r12"
            remote="aosp"
            sync-c="true"
            sync-j="4" />