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If a client wants to enable BLE scanning, it should take a ScanHandle.
When all ScanHandles are destroyed, scanning will be disabled.

This is to avoid clients from fighting with each other to determine if
BLE scanning should be enabled or not.

Bug: b/110432359
Test: cast_bluetooth_unittests, device_unittests, manual.
Change-Id: I23dbded8a5e1bd2f687b70d5d594140b4a23df8e
Reviewed-on: https://chromium-review.googlesource.com/1112585
Commit-Queue: Bailey Forrest <bcf@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570564}