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/common/dynamic_partition_control_stub.h b/common/dynamic_partition_control_stub.h
index eb7154c..515ec7c 100644
--- a/common/dynamic_partition_control_stub.h
+++ b/common/dynamic_partition_control_stub.h
@@ -64,9 +64,12 @@
       const std::string& unsuffixed_partition_name,
       const std::optional<std::string>&,
       bool is_append) override;
-  FileDescriptorPtr OpenCowReader(const std::string& unsuffixed_partition_name,
-                                  const std::optional<std::string>&,
-                                  bool is_append = false) override;
+
+  FileDescriptorPtr OpenCowFd(const std::string& unsuffixed_partition_name,
+                              const std::optional<std::string>&,
+                              bool is_append = false) override {
+    return nullptr;
+  }
 
   bool MapAllPartitions() override;
   bool UnmapAllPartitions() override;