AU: Try delta updates first, then full updates

src/platform/update_engine/update_metadata.proto

Index: src/platform/update_engine/update_metadata.proto
diff --git a/src/platform/update_engine/update_metadata.proto b/src/platform/update_engine/update_metadata.proto
index 61b496513d8dfa69c8371d8aef337f5d2202c9e2..42880dae3b2062cb66ffb32b840c6a1cb1e652dc 100644
--- a/src/platform/update_engine/update_metadata.proto
+++ b/src/platform/update_engine/update_metadata.proto
@@ -102,6 +102,8 @@ message DeltaArchiveManifest {
     }
     // If present, there is data associated with this File object and
     // data_offset and data_size must be set.
+    // If a file object doesn't have this set, it means the data is
+    // unchanged from the old version of the file.
     optional DataFormat data_format = 4;
     // The offset into the delta file where the data (if any) is stored
     optional uint32 data_offset = 5;