Make dynamic partition control android return a writable fd

We can return a FileDescriptor object, which encapsulates logic needed
to write to a COW. This way, filesystem verfication action can use the
turend value directly for computing verity and hash the partition.

Test: th
Change-Id: Iafe9699ef0cc15961641fc94f8ad2820230a56e1
diff --git a/payload_consumer/filesystem_verifier_action.cc b/payload_consumer/filesystem_verifier_action.cc
index aa2fbaa..4efbe41 100644
--- a/payload_consumer/filesystem_verifier_action.cc
+++ b/payload_consumer/filesystem_verifier_action.cc
@@ -129,8 +129,8 @@
   const InstallPlan::Partition& partition =
       install_plan_.partitions[partition_index_];
 
-  read_fd_ = dynamic_control_->OpenCowReader(
-      partition.name, partition.source_path, true);
+  read_fd_ =
+      dynamic_control_->OpenCowFd(partition.name, partition.source_path, true);
   if (!read_fd_) {
     LOG(ERROR) << "OpenCowReader(" << partition.name << ", "
                << partition.source_path << ") failed.";