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1. Broker to take path in constructor.
2. Broker to create local InterfacePtr for ExternalConnector.

These changes support testing external mojo interfaces outside of
service manager, and with multiple instances.

Bug: internal 138940183
Test: Verified while testing external service.
Change-Id: I367495003c63daf5b1c701af343d8e8871646b18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1737041Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Chris Letnick <cletnick@google.com>
Auto-Submit: Chris Letnick <cletnick@google.com>
Cr-Commit-Position: refs/heads/master@{#684229}