Implement better handling of text in assist.

TextView is now much smarter about the text it reports, limiting it
to what is visible (plus a bit more).  Also add a facility for it to
report where the lines of text are, both as offsets in the text string
and their baselines on screen.

Part of fixing issue #22328792: Fix scalability issues in AssistStructure

Change-Id: Idddb8c3a3331355f381e2d4af06d520fe7c7ce8e