Add API to block focus in the presence of a touchscreen

For the sake of devices with touchscreens and optional keyboards, add
the touchscreenBlocksFocus attr and associated get/set methods to
ViewGroup. This will act much like FOCUS_BLOCKS_DESCENDANTS, but only
if the context reports that a touchscreen is present. This allows an
app to define much coarser-grained block elements for focus instead of
navigating between each individual (normally) focusable element, on
the theory that the readily available touchscreen or other keyboard
shortcuts allow for more fine-grained interaction. Keyboard focus
navigation thereby becomes more efficient at a coarse level.

Bug 13987814

Change-Id: Ie652b8845122a59046e96ad6074b3de163779adc