Prioritize system toasts

 Insert toasts from system packages at the front of the queue
  to ensure that apps can't spam with toast to delay system toasts from showing.
 Also increase Clipboard paste warning toasts length to LENGTH_LONG.

Test: atest NotificationManagerServiceTest
Bug: 293301736
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:58d89b491668663963e66906196fd93b9c73ee80)
Merged-In: I13547f853476bc88d12026c545aba9f857ce8724
Change-Id: I13547f853476bc88d12026c545aba9f857ce8724