Add superproject directive to studio-master-dev manifest

This makes it possible to use "repo init --use-superproject" to
work on Android Studio using the superproject at
https://cs.android.com/android/platform/superproject/+/studio-master-dev:;bpv=1
to determine which revisions to sync to (speeding up syncs).

Bug: 179696378
Change-Id: Ibcef12a02f980366250b9edd09461e051ae08294

default.xml

diff --git a/default.xml b/default.xml
index dbeebaf..e9cba28 100644
--- a/default.xml
+++ b/default.xml
@@ -7,6 +7,7 @@
   <default revision="studio-master-dev"
            remote="aosp"
            sync-j="4" />
+  <superproject name="platform/superproject" remote="aosp" />
 
   <!-- BEGIN open-source projects -->
   <project path="tools/buildSrc" name="platform/tools/buildSrc">