floss: Fix racing in start/stop scan

Consider the following race:
1. The 1st StartScan is called with filter:
     has_active_unfiltered_scanner = false
2. The 2nd StartScan is called without filter:
     has_active_unfiltered_scanner = true
3. Async task of the 2nd StartScan is done -> Unfiltered scanning mode
4. Async task of the 1st StartScan is done -> MSFT scanning mode
==> Want unfiltered scan, but got MSFT scan

This patch move the decision into the async tasks, so it is protected
by the lock and the real executing order doesn't matter.

Bug: 279233527
Tag: #floss
Test: Manual test
Change-Id: Ib41d2bbf965a81edce23d62ae6df9832362fc1af