leaudio: Improve TA reconnection

With this patch, LeAudio tries to keep TA background connect registered
even device is already connected.
This is to avoid possible race, when there are other applications with
regular background connect enabled. Race happens on ACL disconnection
and might lead to unwanted reconnecet happen due to device being in the
allow list before Le Audio enabled TA again.

Bug: 311578937
Test: atest bluetooth_le_audio_client_test
Tag: #feature

Change-Id: Ifb7bf8c2a27921649b5ceb43900bf20385c91588