Unhide some methods in java.util.concurrent.CompletableFuture

Methods are stable and haven't been changed since 9 to 15:
 * completeAsync(Supplier<? extends T>, Executor)
 * completeAsync(Supplier<? extends T>)
 * completeOnTimeout(T, long, TimeUnit)
 * completedStage(U)
 * copy()
 * defaultExecutor()
 * delayedExecutor(long, TimeUnit, Executor)
 * delayedExecutor(long, TimeUnit)
 * failedFuture(Throwable)
 * failedStage(Throwable)
 * minimalCompletionStage()
 * newIncompleteFuture()
 * orTimeout(long, TimeUnit)

Bug: 179233422
Test: m update-api && m droid
Change-Id: I824fe64a2ba40e15a8732ba4571a5d81f488e19f
2 files changed