Dynamically enable spatial navigation based on events

As currently implemented spatial navigation and touch event handling
have some unexpected interactions (see bug link) so instead:
1/ make a better-guess initial value based on presence of touchscreen
2/ dynamically enable spatnav when a DPAD event is received, and disable
   it when touch down event received.

BUG=309425

Review URL: https://codereview.chromium.org/33823005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230304 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230304