leaudio: Improve handling lack of active devices.

State machine sometimes checks for active devices. If there is non,
which should not happend :), then StopStream was called. This was wrong,
as there is no active device. Instead group shall be cleared and
IDLE state reported up if needed.

Bug: 265501806
Test: atest bluetooth_le_audio_test
Test: atest BluetoothInstrumentationTests
Tag: #feature
Change-Id: Ic50fca7745f3846bdf55f832b8b41834308990be