[Tizen] Add unit test for app-widget configuration element handler

This patch adds tizen_appwidget_handler_unittest.cc. New tests verify
that the handler properly handles the following configuration elements:

widget.app-widget @id @primary @update-period @auto-launch
widget.app-widget.box-label @lang #text
widget.app-widget.box-icon @src
widget.app-widget.box-content @src @mouse-event @touch-effect
widget.app-widget.box-content.box-size @preview @use-decoration #text
widget.app-widget.box-content.pd @src @width @height

BUG=XWALK-1515, XWALK-1516
hm.. I did not see this in previous patches. Why do you need an extra 'key' variable?
Sorry for being annoying but unneeded std::string allocations might lead to big problems: https://groups.google.com/a/chromium.org/forum/#!msg/chromium-dev/EUqoIz2iFU4/kPZ5ZK0K3gEJ