docs: Fixing badly formatted {@code} strings

{@code} blocks in .jd files automatically escape special characters
(like <>&) in the contents, which is bad if the contents were
*already* escaped--it turns (e.g.) "&lt; foo &gt;"" into
"&amp;lt; foo &amp;gt;". Fixing it where I find it, by replacing
{@code} tags with old-fashioned <code></code>.

See first comment for doc stage location.

bug: 30038013
Change-Id: Ic502096dc69fb7b43f58a34c3f92fd3a41f033f6