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- NearbyConnection is owned by NearbyConnectionsManager, and is
  destroyed when it's disconnected by either end from various methods
- NearbyConnection offers a disconnection listener to listen for
  destruction
- NearbyServiceManagerImpl expects to overwrite NearbyConnection's
  disconnection listener time to time, but NearbyConnectionImpl
  currently allows multiple listeners, causing
  NearbyServiceManagerImpl's disconnection handler to be called multiple
  times
- Changed NearbyConnectionImpl's disconnection listener back to a single
  member that can be overwritten
- Made IncomingFramesReader owned by IncomingShareTargetInfo so it'll be
  cleaned up in UnregisterShareTarget

Bug: 1076008
Change-Id: I16effec3526e0e55a958a96bc3893f5d619e5a1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339982Reviewed-by: Richard Knoll <knollr@chromium.org>
Reviewed-by: Himanshu Jaju <himanshujaju@chromium.org>
Commit-Queue: Alex Chau <alexchau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795970}