[DO NOT MERGE] Catch a missed case when adding a toast

We allow a single toast window per UID. It is possible that
the UID had another toast window that is going away and this
CL adds a missed check for when a window is scheduled to be
removed.

bug:31340854

Change-Id: I0621516019542c85fe7121f7fe61659e4d4eb672
2 files changed