[Android] Implement loading app from manifest file.

When the Java method AppFromManifest() in Java class XWalkView is
called, the manifest string will be passed to c++ side, it will be
parsed by the manifest parser. If a valid target URL could be got,
it will be passed back to the Java side, the Java class XWalkContent
will navigate to target page with the URL.

To comunicate between C++ and Java, the following interfaces are
added to XWalkContent.java:
1. The native method nativeSetManifest().
2. The Java method onGetUrlFromManifest() called by native.