Revert "unique_fd: easier to forward declare"

This reverts commit cbe250db6c6a617415ed53e6ebb265cfdee780f3.

Reason for revert: Failed TreeHugger, seemed to introduce presubmit regression

Bug: 186667911
Change-Id: Ia74635f7ce335485f01fe3ada9dd586f58c97706
diff --git a/include/android-base/unique_fd.h b/include/android-base/unique_fd.h
index 6d89c07..9ceb5db 100644
--- a/include/android-base/unique_fd.h
+++ b/include/android-base/unique_fd.h
@@ -85,7 +85,7 @@
 };
 
 template <typename Closer>
-class unique_fd_impl {
+class unique_fd_impl final {
  public:
   unique_fd_impl() {}
 
@@ -175,9 +175,7 @@
   }
 };
 
-class unique_fd final : public unique_fd_impl<DefaultCloser> {
-  using unique_fd_impl::unique_fd_impl;
-};
+using unique_fd = unique_fd_impl<DefaultCloser>;
 
 #if !defined(_WIN32)