Make underlining spans independent of actual class

Previously, we had singled out UnderlineSpan for special treatment
for proper kerning at span boundaries. That had the limitation that
any other span that used underlining, or even subclasses of
UnderlineSpan were not getting the fix.

With this CL, we actually look at all the effects of applying a
span's updateDrawState() and if the changes were only in underlining,
we merge the pieces and call handleText() on the merged chunk of
text.

Change-Id: I829162c758192b80594f164d1105699224b627ba
Bug: 32907446
Test: cts-tradefed run cts-dev --module CtsTextTestCases
Test: cts-tradefed run cts-dev --module CtsWidgetTestCases
Test: manual