Only allow using PendingRemAnimRegistry on top launched activity

When launching multiple activities, we only allow using the
PendingRemoteAnimationRegistry on the top activity. Otherwise, the
pending entry will be consumed on the first activity, but that one
is not actually used to determine the animation.

Test: Open Google+ notification, ensure correct notification
animation
Test: go/wm-smoke
Fixes: 77855085

Change-Id: I9b86fc283335558c3ce92e498b232bb26dcd1eb4