Update list of mainline-involved files

This is used by repo hooks to remind uploaders of files that should
also be uploaded to mainline branches. Also provide OWNERS info to ease
future updates to the file.

Bug: 247992660
Bug: 245354160
Test: manual
Change-Id: I9c8940bee1dcb6a8ba0724854a243c04c921a529
diff --git a/OWNERS b/OWNERS
index 87bc809..3c7a3ab 100644
--- a/OWNERS
+++ b/OWNERS
@@ -10,3 +10,7 @@
 ## Only contact for media changes as a fallback
 etalvala@google.com
 shuzhenwang@google.com
+
+# mainline related
+per-file MainlineFiles.cfg=essick@google.com
+per-file MainlineFiles.cfg=file:/media/janitors/reliability_mainline_OWNERS