Prevent early recent trimming of distinct documents.

Prevent early trimming of documents with distinct
intents. Otherwise when launching a document
from a given activity, existing recent entries
will always be pruned even if they are distinct
documents. We don't have to worry about overflowing
recents, a seperate codepath (from addLocked)
takes care of second pass trimming when we have
exceeded our total max recents.

Bug: 25460953
Change-Id: I71f8c3d46570df2c999f0d986e05515074e0a098