Tweak toHtml() paragraph conversion behavior

For mode TO_HTML_PARAGRAPH_LINES_INDIVIDUAL:
1) Empty line outputs <br> instead of <p><br></p>

2) A <ul> element is closed if it is followed by a <br>

3) Explicit zero vertical margin is applied to <ul> instead of <li>'s

4) More deterministic way to encode AlignmentSpans: only spans with
   SPAN_PARAGRAPH flag is used. If multiple spans exist, the last one
   (which is added most recently) is used.

BUG: 26224878
Change-Id: I86a2aeced9965ae465daac1ace64e5e41cf45caf