WASHINGTON — Serving in the same platoon role as he has for the past two seasons, Austin Slater simply hasn’t gotten many opportunities as the Giants faced a stretch of seven straight right-handed starters to begin this road trip. The platoon role took some adjustment for Slater, a former prospect who was used to regular opportunities against pitchers of any handedness. He called last season — his first taking more at-bats against lefties than righties — a learning experience. One thing he picked up: stay ready for the next southpaw. So he had ample time to prepare for the Giants’ series opene…