RFR: JDK-8280393: Promote use of HtmlTree factory methods

[Hannes Wallnöfer]

This is a simple cleanup to add and/or make use of `HtmlTree` factory methods for `UL` and `DIV` with a single `HtmlStyle` parameter.

There are no test changes (noreg-cleanup) and no changes in the generated JDK documentation.

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Commit messages:
 - JDK-8280393: Promote use of HtmlTree factory methods

Changes: https://git.openjdk.java.net/jdk/pull/7162/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7162&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280393
  Stats: 108 lines in 19 files changed: 11 ins; 40 del; 57 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7162.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7162/head:pull/7162

PR: https://git.openjdk.java.net/jdk/pull/7162