[19/n] Letterbox Education: fix multi-user preference key

TaskInfo#userId should be used instead of Context#getUserId because the
latter isn't correct following user switch.

In addition, don't mark the education as seen for the current user until
the dialog enter animation is done, to make sure the dialog is fully
visible to the user.

Fix: 225840105
Test: atest WMShellUnitTests:LetterboxEduWindowManagerTest
Change-Id: Ic99c59bc866735bdc1420513f7a3eade4253e391