libsnapshot: set header max_compression

Intermediate CL needed before variable block size can land. Since v3 is
enabled on cuttlefish, the base build needs to write the
compression_factor in order for reader to properly parse. Otherwise
we'll fail OTA test

Test: th
Change-Id: Ia353aae8e668858851073f09308909ae70d7854e

fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp

@@ -110,6 +110,7 @@ void CowWriterV3::SetupHeaders() {
     header_.op_count = 0;
     header_.op_count_max = 0;
     header_.compression_algorithm = kCowCompressNone;
+    header_.max_compression_size = options_.compression_factor;
     return;
 }