Add accessibility action for scrolling to a collection position

Cleans up verbose R package in GridView, accessibility action docs,
and some lint warnings in AccessibilityNodeInfo. Makes the action IDs
for SHOW_ON_SCREEN and SCROLL_TO_POSITION public so that we can
guarantee they won't change (and thus reference them in support lib).

Change-Id: Ica53b7be7a68b84054b9bac1fc7958a21a42e089