Adds content description when there is no result.

Bug: 30630099
Test: Manually tested 1) tapping the result box on cold start, 2)
tapping the result box on warm start, 3) tapping the result box when a
formula is entered but no result, 4) tapping the result box when there
is a result, 5) scrolling the result, and 6) after an existing result is
deleted, both with delete and clear. Unit tests also still pass.

Change-Id: Ief31dc0417592b17ea9a8c6c64a35928aead1ea7