[Audiosharing] Refine the stats log with nonnull stats key

To differentiate the stats log from some framework generated log with
null changedPreferenceKey.

Flag: com.android.settingslib.flags.enable_le_audio_sharing
Test: atest
Bug: 305620450
Change-Id: I9d9bf2aacee0c71ab3b2a3aaecec22acdfc54d31