Fix the HTML escaping of typed text.

Replace “`<`”, “`>`”, “`&`”, “`"`” and “`'`” with HTML entities by hand for API < 16, where there is no `Html.escapeHtml()`
 (Those five characters are the “htmlspecialchars” in PHP. That list should be good enough.)

Seems to fix the [recent crashes](https://groups.google.com/forum/#!topic/anki-android/ajy_5TWacVM).