Add Promise#andFinally and Promise#isPending

- andFinally: Inspired from JS's Promise.finally, see
  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/finally

- isPending: Since base's Java Promises need to be resolved
  via synchronous calls and trigger errors instead of ignoring attempts
  to fulfill or reject a settled Promise, the state checks are very
  helpful. This adds the missing one for the state when the Promise
  is not fulfilled nor rejected.

Change-Id: I7b58cc555ce3339ba3e325fb5e3b83e8df138ce0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5772759
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1339614}