It's essential not to apply a break opportunity within a replacement
span, otherwise things can happen such as displaying the span twice.
The old code tested this case based on zero-width characters.
However, this test was both imprecise, and also in some cases read
uninitialized values from the mCharWidths array, which in turn led
to inconsistent line breaking of the same text.

This patch applies all line break opportunities (as identified by
ICU) within text (as opposed to replacement spans), and also applies
break opportunities at the beginning and end of replacement spans,
but avoids breaks within a replacement span.

Bug: 20138621
Change-Id: I36baeb44d6808356649e1bb69ca57f093fc8c723