Only notify about clipboard access once per clip per app.

This limits the toast notification "<app> pasted from <source>"
to be shown only once per clip per app/user.

Bug: 171640049
Test: Manual, copy from one app then paste multiple times in another.
Check that other apps still display message the first time when pasting.

Change-Id: Ida66bce0301116e0236e0acced6550cafa13ac39