add PREUPLOAD.cfg for a media-related external repo

Bug: 157501573
Test: commit/repo upload
Change-Id: I7cd8fb7449fe1402c3501570262586d3bcdeff59
diff --git a/PREUPLOAD.cfg b/PREUPLOAD.cfg
new file mode 100644
index 0000000..ecf8b8e
--- /dev/null
+++ b/PREUPLOAD.cfg
@@ -0,0 +1,2 @@
+[Hook Scripts]
+mainline_hook = ${REPO_ROOT}/frameworks/av/tools/mainline_hook_project.sh