bthal: Do not allow to initialize more than once

Because the Bluetooth HAL service can serve one client at a time, but is
stuck by double attempting, here we check if there is a transport layer,
and reject the second request.

Bug: 169472630
Test: VTS
Tag: #stability
Change-Id: I5046eaa0bcc9b7f16713b3940af4a73e56ea3630