leaudio: Handle OnGattConnect for already connected device

This is minor regression after
35c5e00d leaudio: Improve TA reconnection

Basically when calling Open for TA in the OnGattConnected function will
trigger OPEN event from GATT, which should be ignored.

Fixed implementation uncovered issue in one unit test which is fixed
here as well.

Bug: 311578937
Test: atest bluetooth_le_audio_client_test
Tag: #feature
Change-Id: I20dc6029ec07a1a1d649f00c19113601ea5f3aea