Don't let UnderlineSpans affect text width

Previously, text that was partially annotated with an UnderlineSpan
could have a different width than text without it, since underlined
text was separated into a separate piece of text and was measured
separately, causing kerning at the span boundaries to disappear.

Change-Id: I118de8524c500fb6a4b05b1bf65fe93dc67f204c
Bug: 32907446
Test: cts-tradefed run cts-dev --module CtsTextTestCases
Test: manual