update_engine: use cow_size_info

Updating update_engine to use a struct called CowSizeInfo which contains
extra parameters needed in v3 of cow format. These fields should be set
in delta archive manifest and passed to snapshot.cpp which will then
use the fields to correctly configure the cow with a correctly sized
operation buffer space.

Test: ota on cuttlefish
Change-Id: I8306bf3e4bbc3fcd8f2e31be116fa0623022c227
diff --git a/update_metadata.proto b/update_metadata.proto
index 3881464..535f386 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -316,6 +316,10 @@
   // as a hint. If set to 0, libsnapshot should use alternative
   // methods for estimating size.
   optional uint64 estimate_cow_size = 19;
+
+  // Information about the cow used by Cow Writer to specify
+  // number of cow operations to be written
+  optional uint64 estimate_op_count_max = 20;
 }
 
 message DynamicPartitionGroup {