LeAudio: Fix error on updating config after session is stopped

Don't update audio configuration when the session is not running.

Fixes the following error logs:
 BTAudioProviderStub: updateAudioConfiguration - SessionType=LE_AUDIO_HARDWARE_OFFLOAD_DECODING_DATAPATH has NO session
 bt_stack: [ERROR:client_interface_aidl.cc(342)] UpdateAudioConfig: BluetoothAudioHal failure: Status(-3, EX_ILLEGAL_ARGUMENT): ''

Bug: 320275987
Test: m com.android.btservices
Flag: EXEMPT; trivial change
Change-Id: Ibe29cce0bffac352447b9d00bf47b66651b16517