Suppress OptionalOfRedundantMethod error

Errorprone is being updated from 2.7.1 -> 2.8.1, which
adds a OptionalOfRedundantMethod check, which has default
severity level ERROR.

Suppress it here as it's part of a test of Optional.

Bug: 190944875
Test: Patch aosp/1792988, then `m RUN_ERROR_PRONE=true javac-check`
Change-Id: I8a7152be0e2ac24eda5d377ef5cfef102487a5dc