Fix testEllipsizeCallback()

An ellipsize test was failing because it made assumptions about the
width of the base string, which weren't correct because "r" and the
ellipsis character are a kern pair in Roboto. This change just computes
a width without the kerning.

Change-Id: I2d7621089f603d5ad6f9544f0a54de4f9a7f9910