Mark fallback owners as #{LAST_RESORT_SUGGESTION}.

These top-level owners are only designed to offer fallback support
in situations such as local OWNERS files becoming stale, large-scale
changes, etc.

When local owners have been defined by a sub-team, only those owners
should appear in the Gerrit suggestions list.

Bug: 181661570
Test: none
Change-Id: Ib201cb32e97707d35df077128f4c5e8205124b7f