GitLab

- Used NearbyFileHandler to properly delete PayloadPtr
- Added base::ScopedDisallowBlocking to the test to ensure the class
  won't use blocking code except in thread pool
- Moved creation of input payload file into NearbyFileHandler
- Added various variable clearign to NearbyConnectionsManagerImpl::Reset

Bug: 1076008
Change-Id: I2c35ddb3d01ae23186416b0175fbdb48d295251e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367682
Commit-Queue: Alex Chau <alexchau@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800710}