Remove line3 in big notification unless there's text to show.

If you have no subText or summaryText in a big template, but
you *do* have a number, the overflow bar (below the big text
or inbox or whatever) would have shown; now it does not.

Bug: 6657006
Change-Id: Ib2af2712da3a98227bd8d697560893adbdc427e9