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Replacing calls to global device_emulator.bluetoothSettings methods with
WebUI events and FireWebUIListener().

This is in preparation of migrating this page to Polymer3.

Bug: 1015241
Change-Id: I3d8bc7f5b008e734670393c0832f741f13195e1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868089Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707396}