drm_hwcomposer: Rework autofd

Motivation:

Current implementation of UniqueFd can be used in a different ways,
making analytical tracking of FD lifecycle much harder than it may be.
Keep this part clean is very important, since any wrong code may open
a hard-to-detect runtime bugs and fd leaks, which may accidentally slip
into the production.

Implementation:

1. Combine UniqueFd anf OutputFd into single class.
2. Reduce the API to be minimal and sufficient.
3. Document the API and use cases.
4. Move to utils/UniqueFd.h.
5. dup(fd) was replaced with fcntl(fd, F_DUPFD_CLOEXEC)) to
   address clang-tidy findings. Find more information at [1]

[1]: https://clang.llvm.org/extra/clang-tidy/checks/android-cloexec-dup.html

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/compositor/DrmDisplayCompositor.h b/compositor/DrmDisplayCompositor.h
index aebb6ff..c0eed0c 100644
--- a/compositor/DrmDisplayCompositor.h
+++ b/compositor/DrmDisplayCompositor.h
@@ -72,10 +72,11 @@
   void Dump(std::ostringstream *out) const;
   void Vsync(int display, int64_t timestamp);
   void ClearDisplay();
-  int TakeOutFence() {
-    if (!active_composition_)
-      return -1;
-    return active_composition_->take_out_fence();
+  UniqueFd TakeOutFence() {
+    if (!active_composition_) {
+      return UniqueFd();
+    }
+    return std::move(active_composition_->out_fence_);
   }
 
   FlatteningState GetFlatteningState() const;