[Nearby] Background scan before onboarding.

This updates the InvalidateBackgroundScanning() logic so that we will
start background scanning even if the user has not completed onboarding.
However, if the user has explicitly disabled Nearby Share we will not
perform background scanning.

Prototype: This code is in a prototype state, and thus omits unit tests.
These changes are behind a feature flag and will be productionized in a
future update.

Bug: 1223888
Change-Id: I38e8d83aaa77302fd45d148306d39bdb46ed598b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3003009
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Reviewed-by: Claude van der Merwe <cvandermerwe@google.com>
Reviewed-by: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#899369}