Enable update_engine to access OTA package via file descriptor

Due to the restriction of Treble, update_engine cannot access to OTA
packages located on non-core domain area.
(e.g. /data/vendor/upgrade/xxx.zip)
To solve such problem, update_engine needs to have a new interface
which accepts a file descriptor (FD) of OTA package file instead of
its URI and to read package file while updating via FD.

Test: Manual update
Bug: 130209137
Change-Id: Ieb7173dc958ba3eb21af708e616ef7078cd17b3e
diff --git a/common/utils.cc b/common/utils.cc
index 34d97a2..e7b6975 100644
--- a/common/utils.cc
+++ b/common/utils.cc
@@ -1064,6 +1064,16 @@
   }
 }
 
+string GetFilePath(int fd) {
+  base::FilePath proc("/proc/self/fd/" + std::to_string(fd));
+  base::FilePath file_name;
+
+  if (!base::ReadSymbolicLink(proc, &file_name)) {
+    return "not found";
+  }
+  return file_name.value();
+}
+
 }  // namespace utils
 
 }  // namespace chromeos_update_engine