LeAudio: Minor NumOfConnected() method cleanup

Split the NumOfConnected() method in two for better readability.

Bug: 308428860
Bug: 295972694
Test: atest --host bluetooth_le_audio_test bluetooth_le_audio_client_test  bluetooth_test_broadcaster bluetooth_test_broadcaster_state_machine bluetooth_le_audio_codec_manager_test --disable-upload-result --minimal-build --no-metrics --no-bazel-mode
Flag: EXEMPT; mechanical refactor; no functional change - confirmed with unit tests
Change-Id: Ie12c0331f4624e7de9d4aa3ce03b743937e01dc9