Reduce log level of log line in BackAnimationController

The log line in question is expected to be triggered in cases when the app has not opted into predictive back. Therefore the level is reduced to DEBUG.

Bug: 334634203
Test: Manual, i.e. verifying that the code path is valid when predictive back is not opted in by app.
Change-Id: Idd727649706ec021100378e6ccad9d5de92d9267