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: If2ec9e8868c8d4a2641f9229f47196bf71bc0981