Add a "vabc_enabled" flag to update_metadata

When doing a downgrade build, the target build might not support vabc.
So disable vabc in this case.

Test: treehugger
Change-Id: Ie5a56adf99b495a40d1caafbda6466181da1bc10
diff --git a/update_metadata.proto b/update_metadata.proto
index 452b89d..c57e4c1 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -348,6 +348,12 @@
   // partitions if possible. If this is unset, the update_engine daemon MUST
   // NOT create snapshots for dynamic partitions.
   optional bool snapshot_enabled = 2;
+
+  // If this is set to false, update_engine should not use VABC regardless. If
+  // this is set to true, update_engine may choose to use VABC if device
+  // supports it, but not guaranteed.
+  // VABC stands for Virtual AB Compression
+  optional bool vabc_enabled = 3;
 }
 
 message DeltaArchiveManifest {