Create temporary files in the system's temp directory.

In Brillo, the temporary directory might not be /tmp. This patch uses
base::GetTempDir() instead and removes the hard-coded mentions of /tmp
when creating temporary directories.

Bug: 22024447
Test: `update_engine_client --update --omaha_url=...` mounts /system on the temp directory under /data/local/tmp

Change-Id: Ibb52cae01419511f91bdfbf6f228b74a581edde9
diff --git a/postinstall_runner_action.cc b/postinstall_runner_action.cc
index 184d66d..c6879d8 100644
--- a/postinstall_runner_action.cc
+++ b/postinstall_runner_action.cc
@@ -48,8 +48,8 @@
   ScopedActionCompleter completer(processor_, this);
 
   // Make mountpoint.
-  TEST_AND_RETURN(utils::MakeTempDirectory("/tmp/au_postint_mount.XXXXXX",
-                                           &temp_rootfs_dir_));
+  TEST_AND_RETURN(
+      utils::MakeTempDirectory("au_postint_mount.XXXXXX", &temp_rootfs_dir_));
   ScopedDirRemover temp_dir_remover(temp_rootfs_dir_);
 
   const string mountable_device =