Adding rank column in backup

> targetType was only added for M builds, so it can also be considered
  as v4 change. V3 which introduced minSpanX and minSpanY didn't
  contain targetType

Bug: 22417713
Change-Id: I5c353674f7e0c2b5b4ab46e574fdb347d82028cd
diff --git a/protos/backup.proto b/protos/backup.proto
index d8d94e8..6704e08 100644
--- a/protos/backup.proto
+++ b/protos/backup.proto
@@ -101,7 +101,10 @@
   optional string iconPackage = 16;
   optional string iconResource = 17;
   optional bytes icon = 18;
+
+  // Added in backup version 4
   optional TargetType targetType = 19 [default = TARGET_NONE];
+  optional int32 rank = 20;
 }
 
 message Screen {
@@ -121,6 +124,7 @@
   optional Resource icon = 4;
   optional Resource preview = 5;
 
+  // Added in backup version 3
   // Assume that a widget is resizable upto 2x2 if no data is available
   optional int32 minSpanX = 6 [default = 2];
   optional int32 minSpanY = 7 [default = 2];