Replace IS_DEBUGGABLE with isDebuggable module API

Introduce a Build.isDebuggable() getter instead of the IS_DEBUGGABLE
static member.

Also make it module API as APK modules now have access to module APIs in
S, but also TestApi so it can be tested.

Test: m
Fixes: 182345786
Change-Id: I25d0848aee758792bbaba8ab73b5dd997a7250b3

services/core/java/com/android/server/ConnectivityService.java

@@ -7677,7 +7677,7 @@ public class ConnectivityService extends IConnectivityManager.Stub
         }
 
         void addRequestReassignment(@NonNull final RequestReassignment reassignment) {
-            if (Build.IS_DEBUGGABLE) {
+            if (Build.isDebuggable()) {
                 // The code is never supposed to add two reassignments of the same request. Make
                 // sure this stays true, but without imposing this expensive check on all
                 // reassignments on all user devices.