leaudio: Improve connecting set members

When one set member got (re)connected, Android adds other set members to
controller allow list for quick reconnection.
For this LeAudio code was using BTM_BLE_BKG_CONNECT_ALLOW_LIST, which
turns out not optimal solution as stack is using slow scan interval and
window to create such connection.

Therefore this change which will start with using BTM_BLE_DIRECT_CONNECTION
which will assure fast scan parameters for connection. In case of set
members being not available withing 30 sec, LeAudio code will fallback
to BTM_BLE_BKG_CONNECT_ALLOW_LIST

Bug: 303247201
Test: atest bluetooth_le_audio_client_test
Tag: #feature
Change-Id: Ic624969d01a34fe32c18df508abecac00eec897a