Use raw pointer instead of shared_ptr

Throughout update_engine, there are many places we pass a shared_ptr of
FileDescriptor, for memory safety. But in many cases, functions only
need to use the pointer througout execution of the function, and do not
need to store these pointers. In these cases, it's perfectly safe to use
a raw pointer. Furthermore, the overuse of shared_ptr make life time of
FileDescriptors unpredictable. This is harmful in context of VABC, where
presence of a file descriptor can cause UnmapAllPartitions() to fail.
Therefore, we switch some of the safe usecases to raw pointer.

Test: th
Bug: 216391325
Change-Id: I3d986688e81fcc1d761b7f4926a3c81ac691788a
diff --git a/common/dynamic_partition_control_interface.h b/common/dynamic_partition_control_interface.h
index a5be6e1..e6ebe6a 100644
--- a/common/dynamic_partition_control_interface.h
+++ b/common/dynamic_partition_control_interface.h
@@ -167,7 +167,7 @@
       bool is_append = false) = 0;
   // Open a general purpose FD capable to reading and writing to COW. Note that
   // writes must be block aligned.
-  virtual FileDescriptorPtr OpenCowFd(
+  virtual std::unique_ptr<FileDescriptor> OpenCowFd(
       const std::string& unsuffixed_partition_name,
       const std::optional<std::string>&,
       bool is_append = false) = 0;