Tweaks to <p> and <li> sep constraints when surrounded by HTML nodes

* With HTML like "<p>a</p>\n<p stx:html>b</p>", we dont need two
  newlines between "a" and "<p>b</p>" on serialization.
  "a\n<p>b</p>" is acceptable.

* Similarly for <li> nodes.

* This reduces selser failures in the big separator handling patch
  that continues to still be WIP.

Change-Id: Iff239e616608c28dcb7eefc7d1c4a227a07b6203
Merge "Tweaks to <p> and <li> sep constraints when surrounded by HTML nodes"