chromeos: Move events related code into its own directory.

Move the event-rewriter code, system key-event listener, and xinput device
event listeners from chrome/browser/chromeos into
chrome/browser/chromeos/events/

BUG=none
R=derat@chromium.org, stevenjb@chromium.org
TBR=for #include path update in chromeos/login/, nkostylev@chromium.org

Review URL: https://codereview.chromium.org/165723004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251634 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251634