leaudio: Fix sending current config to Java

Fix initial value to btle_audio_codec_config_t, sample rate translation
and make sure current config can be send even stream is not yet
configured. For the last point, use latest way of getting configuration
instead of using group->stream_conf which might be invalid.

Bug: 314094343
Test: mmm packages/modules/Bluetooth
Test: atest bluetooth_le_audio_client_test
Flag: Exempt, trivial change, regression tested with unit test, new test
added
Change-Id: I4785dd95e74a44bb73674dcc7e70cd43bfea395c