[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.
Refer to http://androidxref.com/4.2.2_r1/xref/frameworks/base/core/java/android/content/pm/PackageParser.java#568, seems  RSA and DSA should be both supported.

I'm okay that we only support RSA keystone so far, but please leave a TODO.