[Android] Use reflection layer in xwalk core for runtime usage 

As now there is reflection layer between xwalk core and xwalk
core internal, there is no need to use the one between runtime
and runtime client for runtime usage.

After this patch, most facilities to support shared mode will be
moved from runtime to xwalk core. Besides, runtime_java will be
merged into runtime_client_java.
XWalkRuntimeLib.apk will keep the name unchanged, but its package
will be org.xwalk.core which was org.xwalk.runtime.lib previously.