Reenable advertising instance after an LE advertising state change

Ensure BleAdvertisingManager::OnAdvertisingStateChanged() is called
after an advertising state change to re-enable advertising after
a connection.

This patch also fixes the registration test, which was broken since
one more advertising instance was freed for general usage.

Test: net_test_stack_multi_adv native test
Change-Id: I76887d98cb3ec386ebc0f7fad95170b8c18b9116