[mojo-bindings] Prep for new type conversions

Adds some more API features for new bindings types to supported
continued conversion of old types to new types. In particular this
introduces:

- disconnect-with-reason handlers and reset functions
- NullAssociatedRemote() to replace nullptr-constructed
  AssociatedInterfacePtrInfos
- Implicit conversions to/from old and new associated types
- Rename ThreadSafeRemote<T> to SharedRemote<T> since it's more
  than just a thread-safe thing

Bug: 955171
Change-Id: I762e13f7b1f38201f98c0b64963b89203bee5956
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582426
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#653813}
8 files changed