[DO NOT MERGE] Allow multiple toasts for the focused app

We don't allow apps to add multiple toast windows to
prevent an attacker to keep adding the same toast as
a workaround for our measure to ensure toast windows
are removed after a timeout. The may cause backwards
compatibility issue for apps that add multiple toasts.
While we need to fix the security vulnerability it is
desirable to make the fix as backwards compatible as
possible. This change allows the focused app to add
as many toast windows as it wants since they will be
removed after the timeout and once the app is not the
one the user uses it will lose the multiple toast add
capability.

bug:30150688

Change-Id: I2d9295926cb49b5bb80c7af2546872ff8ca22c64
(cherry picked from commit 296a60ac)