Revert "Revert "Fix notification group dismissal fade animation""

This reverts commit eb145298.

Reason for revert: Reverting the "Fix notification group dismissal fade animation" CL did not cause the test metric to recover. Apparently, another CL caused the metric to increase. Therefore, it should be save to merge this change back in.

Bug: 253532643
Test: Manual, i.e. posting a Standard notification and an Inbox notification from the Notify2-RVC App (as well as the Notify2-SC app), then dismissing both notifications together by swiping the group header.
Change-Id: Iadfa1db6630bf01ca3a36977c41366af77e4b263