Remove Network::Type and Network::getType()

Enumeration Network::Type and member function Network::getType() can be
removed after aosp/1657760. Specializations of each type of network are
implemented in subclass. No functionality changes.

Test: atest
Change-Id: Ib57bf26e4f0a60f7f01bec3109fb84458fb3abfb
diff --git a/server/UnreachableNetwork.h b/server/UnreachableNetwork.h
index 09fdf16..a80f3f3 100644
--- a/server/UnreachableNetwork.h
+++ b/server/UnreachableNetwork.h
@@ -29,7 +29,7 @@
     bool canAddUsers() override { return true; }
 
   private:
-    Type getType() const override;
+    std::string getTypeString() const override { return "UNREACHABLE"; };
 };
 
 }  // namespace android::net
\ No newline at end of file