[Android] Expand the path of app_root in LaunchScreen

If the provided path of manifest.json is not absolute, the launch screen image can not be found
and will cause build fail.
Change to use ManifestParser.input_path.

BUG=XWALK-1795
Merge pull request #2014 from kurli/launchscreen_expand_apth

[Android] Expand the path of app_root in LaunchScreen