[Android] Fix resource loading issue of shared mode

Previously, xwalk_runtime_client_shell incorrectly packages
all xwalk resources within it, which hide the resource
loading issue of shared mode.

In commit fbe4a57, attempting to fix this by creating
MixedResources and override "getResources()" of Context.
But in chromium, there exist bunch of code that get resources
from application context.

In this fix, xwalk will provide an overridden Application
which return XWalkMixedResources for getResources().

In such case, all resource loading within XWalkRuntimeLib will
try first loading from library's Context and then from app's
if not found.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1609