Fix reordering of metadata bug

If you had <meta /><b>Annotated text</b> in a wrapper paragraph,
the converter would swap them and output the linear model equivalent
of <p wrapper><b>Annotated text</b></p><meta />.

This happened because the meta item was queued, and annotations didn't
trigger metadata queue flushes. The fix is to trigger a metadata queue
flush whenever we're about to write something that isn't itself queued.

Change-Id: I168abc0392fbec5503271d1653ee5c38518f857d
Merge "Fix reordering of metadata bug"