Update repo hooks for media mainline repos

Change to the R branches; guidance to users when they need to override.
separate the list-of-files for partial repo (frameworks/av) from the script.
Also updates the configuration specific to frameworks/av as a repo split
between mainline/non.

Bug: 157501573
Test: various combinations of repo upload
Change-Id: I726f218c562ea151b4f76ded0903c68400a2f3fd