[KeyguardMediaController] Fix activeContainer logs

We had a field named activeContainer in KeyguadMediaController which is only used for logActiveMediaContainer.
As this field is not calculated field. We see its value is null in logs which might cause us to draw wrong conclusions. This CL fix this.

Bug: 298213983
Test: BR
Flag: NONE
Change-Id: I352bf6c9f8b6d263c8dcb0129af0e43a14013e05