[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
Merge pull request #1905 from kurli/launch-screen-show-bg

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