AndroidPlatform: Gracefully handle unsupported icons in manifest.json
AndroidPlatform: Gracefully handle unsupported icons in manifest.json

When no good icon for android is found (png, jpeg, gif), fall back
to the builtin crosswalk icon with a warning.

BUG=XWALK-4761

crosswalk-project/crosswalk-app-tools,hgl888/crosswalk-app-tools,pozdnyakov/crosswalk-app-tools,shawngao5/crosswalk-app-tools,crosswalk-project/crosswalk-app-tools,baleboy/crosswalk-app-tools,davidmaxwaterman/crosswalk-app-tools,hgl888/crosswalk-app-tools,davidmaxwaterman/crosswalk-app-tools,pozdnyakov/crosswalk-app-tools,baleboy/crosswalk-app-tools,crosswalk-project/crosswalk-app-tools,baleboy/crosswalk-app-tools,shawngao5/crosswalk-app-tools