Notify all observers of a selection change whenever data set changes.

Fixes bug where a downloaded file removes from the selection set but
does not update the "n selected" text.

Bug: 67897012
Test: runtest --path packages/apps/DocumentsUI
--not-annotation=android.support.test.filters.LargeTest

Change-Id: I7d4b7898e3bc85a049947e2de2803325264acd28