ANDROID: GKI DrNo: Do not suggest backup reviewers

When Gerrit is suggesting reviewers either through the UI or through its
APIs for automatic assignment, it does not distinguish between regular
reviewers and backup reviewers. Teach Gerrit the difference.

Bug: 194656797
Bug: 194314089
Signed-off-by: Matthias Maennich <maennich@google.com>
Change-Id: Id96e2b6f9f2317266e55c908f99cc3694fa182c8