Limit Instant App notifications

Instants apps are not allowed to create toasts or notifications, however
they are allowed to update notifications that were created by the
system for them (i.e. a notification for a foreground service).

Test: Manually verified that .notify() fails with a security exception
Test: Manually verified startForeground shows a notification and that it
can be updated.

Change-Id: I054cccc2aa817263d15abd34851b0e06f16fb829