Fix SpannableStringBuilder.replace() exception.

When SpannableStringBuilder.replace is called with a source text that
contains a span with SPAN_PARAGRAPH flag, the code tries to retain the
spans. However when the paragraph boundary constraint is not satisfied,
setSpan method throws an exception. Updated the rule as: if the source
text has a span with SPAN_PARAGRAPH flag check if it can be copied into
the target. If not, discard the span. Also updated the JavaDoc for
Spanned and Editable.

Bug: 22521443
Change-Id: Ie8541e00bfdf5b8b0115ad7b26cb9f83a6a3ee55