Add vdc checkpoint restoreCheckpointPart

Restores the first n entries of a checkpoint. Allows automated testing
of interrupted restores.

Test: vdc checkpoint restoreCheckpoint [device] [n]
Change-Id: I47570e8eba0bc3c6549a04a33600df05d393990b
diff --git a/binder/android/os/IVold.aidl b/binder/android/os/IVold.aidl
index 4b21078..83ee116 100644
--- a/binder/android/os/IVold.aidl
+++ b/binder/android/os/IVold.aidl
@@ -103,6 +103,7 @@
     void commitChanges();
     void prepareCheckpoint();
     void restoreCheckpoint(@utf8InCpp String device);
+    void restoreCheckpointPart(@utf8InCpp String device, int count);
     void markBootAttempt();
     boolean supportsCheckpoint();