[wlan][policy] Location sensor can't block scan results

This relands commit d9f1226af09993c50626196abc1585c65e783359. PS #1
is the same as that commit, but later PSs add fixes for the same
flake described in https://fxrev.dev/781465

Moves sending of scan results to the location sensor into its own
future, such that it doesn't block sending scan results to any
requesters and is instead serviced in parallel.

Bug: 73821
Bug: 76310
Bug: 116134
Test: fx test wlancfg-tests
Multiply: wlancfg-tests
Change-Id: Icddc612be3b835a40baff6742b15f593c1a0251e
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/781466
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Kevin Sakuma <sakuma@google.com>
Fuchsia-Auto-Submit: Marc Khouri <mnck@google.com>