[Android] Register JNI entries while libxwalkcore.so is loaded

Upstream moves RegisterLibraryLoaderEntryHook from content
to base, but the real registration is still done at
content::LibraryLoaded, which is missed in xwalk.

Add the callback back to fix the issue.
Merge pull request #1694 from wang16/next

[Android] Register JNI entries while libxwalkcore.so loaded