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.

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.protos.AddressBook;
-import com.example.tutorial.AddressBookProtos.Person;
+import com.example.tutorial.protos.Person;
 import java.io.BufferedReader;
 import java.io.FileInputStream;
 import java.io.FileNotFoundException;

examples/ListPeople.java

@@ -1,7 +1,7 @@
 // See README.txt for information and build instructions.
 
-import com.example.tutorial.AddressBookProtos.AddressBook;
+import com.example.tutorial.protos.AddressBook;
-import com.example.tutorial.AddressBookProtos.Person;
+import com.example.tutorial.protos.Person;
 import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.PrintStream;

examples/addressbook.proto

@@ -14,7 +14,8 @@ import "google/protobuf/timestamp.proto";
 // [END declaration]
 
 // [START java_declaration]
-option java_package = "com.example.tutorial";
+option java_multiple_files = true;
+option java_package = "com.example.tutorial.protos";
 option java_outer_classname = "AddressBookProtos";
 // [END java_declaration]