DocsUI: Don't animate selection changes on all bind events.

When directory contents update, this causes all items to be bound, which
can cause flickering even though item selection state hasn't changed.

Add a boolean var to trigger UI animations only when selection changes
are triggered by the selection manager.

BUG=27680153

Change-Id: I892d8111cf3ba7e1610d94af4a1a646f1cdb5546