Make a11y text navigation by page symmetric.

Traversals forward and then backward in text didn't land in consistent
places. This fix makes them land in exactly the same place as long as
a page is an integral number of lines long.

Also updating documentation about AccessibilityEvent reported indices
when traversing.

Bug: 27505504
Bug: 27480027
Change-Id: I2c11b6ae61f79d66e5ec101048584b3442be0b42