frameworks/base: Cap the number of toasts that a package can post.

NotificationManagerService keeps track of requested toasts in a
queue. Any package can trigger a DoS by repeated enqueue of
toasts which eventually results in a leak of WeakReferences in
system_server and causes dalvik (hosting system_server) to
abort the same.

Change-Id: I5e23c1bf7e195b07344711d2c6719fa568f2dfaf