[java-spanner] AsyncTransactionManager.CommitTimestampFuture violates Future.get contract

`CommitTimestampFuture` violates the `Future` contract by overriding `get()` to throw any caught exception other than `ExecutionException`. Namely, it unwraps any `AbortedException` and throws that directly.

This breaks any downstream code that operates on that `CommitTimestampFuture` via the `Future` interface. For example, transforming it to a Guava `ListenableFuture` and then calling the `kotlinx.coroutines.guava.await` extension function on it.