Add log for update scan mode during screen on

Nothing is reported or logged when scan mode update happens during
screen on with `updateScanModeScreenOn`. Thus adding a log for debugging
when scan mode changes happen.

Bug: 337976342
Test: m com.android.btservices
Flag: EXEMPT, adding logs only
Change-Id: Iffca6e49c1b108c8c744d675a18581edc6b34d9d