WebView: wrap Java literals with {@code}

Docs change only.

This wraps some Java literals (true, false, null) with {@code} blocks,
as per Android API guidelines.

This also addresses other presubmit errors:

 * fix lines that have become too long
 * fix broken import order and unused imports

Bug: 65213517
Test: make docs (and manually verify things look better)
Change-Id: Idc7fe28d40bea7bd1edcad539b75fa9c689b8d46