Allow mdb/mainline-prod-approvers to vote on GA.

Mainline leads should be able to vote Global-Approval+1 in mainline
module projects across branches. This will be used to approve mainline
LSCs across mainline projects.

Bug: 190142043
Test: presubmit
Change-Id: I86112cec31eb3220328af9ca06a6c06d1f114565