Add platform-systemserverclasspath.

Make it required in platform-bootclasspath, to make sure both are
installed at the same time.

Bug: 180105615
Test: m && launch_cvd
Change-Id: I58fec3781309727e35c5eb53959e7b6ff58fee73

boot/Android.bp

@@ -98,4 +98,12 @@ platform_bootclasspath {
             dest: "hiddenapi-unsupported.csv",
         },
     ],
+
+    required: [
+        "platform-systemserverclasspath",
+    ],
+}
+
+platform_systemserverclasspath {
+    name: "platform-systemserverclasspath",
 }