De-alienate <code>foo</code> blocks (code element)

Adding the <code> element as a matched text style annotation, plus some
tests (for all the other un-tested text styles as well, whilst I'm at
it).

We'll need an icon, a button and a way of the buttons not forever
extending the length of the toolbar to properly edit <code> spans, but
this is a start.

Bonus: unit test coverage for all TextStyleAnnotations

Bug: 51590
Change-Id: I5438bcf2ec6eeb2e50400f8013964f91c33ce455
Merge "De-alienate <code>foo</code> blocks (code element)"