[Android] Show launch screen dialog when no 'image' provided

Previously, when there is no 'image' provided, the launch screen dialog will not be displayed.
It should be displayed, because the backgrounds need to be displayed until the ready_when state.

BUG=XWALK-1635

(Cherry picked from commit d3a5c983c49297fcb629b259861bc4c0a30204e3)