fallback on older cow version

If a device's source or target cow version is specified at 2 (or
undefined) we should fallback on using the v2 writer.

e.g.
1. v3 -> v2 downgrade OTA
2. v2 -> v3

Bug: 322279333
Test: ota_from_target_files
Change-Id: I8c5139eceeb64ee255ddea66017b3e17bbdc44c0

tools/releasetools/common.py

@@ -479,6 +479,10 @@ class BuildInfo(object):
   def vabc_compression_param(self):
     return self.get("virtual_ab_compression_method", "")
 
+  @property
+  def vabc_cow_version(self):
+    return self.get("virtual_ab_cow_version", "")
+
   @property
   def vendor_api_level(self):
     vendor_prop = self.info_dict.get("vendor.build.prop")