getTablePrefix() is used to show the inline legend and inline switcher.
It is not yet part of a released stable interface.
Theoretically there may be multiple text slots on a page, and we don't
want multiple inline legends. There was already a fragment assembly
system, for the benefit of hook handlers, so move that up to the page
level, so that it can also deduplicate prefix fragments coming from
each slot.
Add tests.
Bug: T324759
Change-Id: I9baa5c24128c63bc318ba13e83a024843f4ab15e
