[Android] For shared mode, loading classes and resources seperatedly

To get rid of flag Context.CONTEXT_IGNORE_SECURITY, load classes of
crosswalk library directly from the apk file. In addition, verify
the pubkey of that apk file before trying to load it.
For resources, still get them by createPackageContext. But now it
doesn't need flag for including code and ignore security check
anymore.

TODO(wang16): add the real comparison of pubkey after finalized the
keystore used to publish crosswalk library.
It's not an unknown case, how we sign our library package is fixed. It's a good idea to add the comments here that we might change it here once we finalized that how to publish the crosswalk library.
But we need to change this part of code in future anyway, because we won't know the pubkey's content until the official keystore finalized.