addressbook.proto: Enable java_multiple_files option

And update the java_package value accordingly.
Enabling java_multiple_files is preferred nowadays.
Refs CL 348101638 which updates the tutorial accordingly.
diff --git a/examples/AddPerson.java b/examples/AddPerson.java
index c262ab7..2f6a231 100644
--- a/examples/AddPerson.java
+++ b/examples/AddPerson.java
@@ -1,7 +1,7 @@
 // See README.txt for information and build instructions.
 
-import com.example.tutorial.AddressBookProtos.AddressBook;
-import com.example.tutorial.AddressBookProtos.Person;
+import com.example.tutorial.protos.AddressBook;
+import com.example.tutorial.protos.Person;
 import java.io.BufferedReader;
 import java.io.FileInputStream;
 import java.io.FileNotFoundException;