Add BLE pairing begin dialog

Add dialog before BLE device selection view instructing the user to
begin pairing process.

Bug: 877344
Change-Id: I9f8bf8b011231a20a09ebbb2ab8073792df276ef
Reviewed-on: https://chromium-review.googlesource.com/c/1356122
Commit-Queue: Jun Choi <hongjunchoi@chromium.org>
Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612560}
4 files changed