Fix ConcurrentModificationException in GattService.onScanResult

If a scan client is added or removed while iterating over
mRegularScanClients, a concurrent modification exception will be thrown,
because we use a regular hash map.

Use a ConcurrentHashMap instead to fix this issue.

Bug: 31122137
Change-Id: Icbd427ebbe63adc6b5bd4ee950ef3a874ce6067b
(cherry picked from commit 92d52b44)