Bluetooth: preserve one advertisement slot for GMS core

This brings back the check on the multiple advertisement support before
granting any Bluetooth LE advertiser. In other words, one slot is preserved
for GMS core.

Bug: 74819586
Test: Run an BLE application and verify the number of advertisements
Change-Id: Iac3b47c76c6f699018982a69e9b04a9d8c631bfb