base

Up/down arrow moves to beginning/end of buffer

With a hardware keyboard, using up arrow within the top line should
move to the beginning of the buffer, to better match desktop text
editing expectations, and similarly for down arrow on the last line.
This patch implements that behavior.

Bug: 17385784
Change-Id: Ia23c23c9cc2462558bca9aaffec7d83e284d55e8