add dumpsys -l to list available services

- for people with poor memory like me :)

Change-Id: I60160ca333ac4864bb8328f164cc2e9eba3660fc

cmds/dumpsys/dumpsys.cpp

@@ -39,7 +39,11 @@ int main(int argc, char* const argv[])
 
     Vector<String16> services;
     Vector<String16> args;
-    if (argc == 1) {
+    bool showListOnly = false;
+    if ((argc == 2) && (strcmp(argv[1], "-l") == 0)) {
+        showListOnly = true;
+    }
+    if ((argc == 1) || showListOnly) {
         services = sm->listServices();
         services.sort(sort_func);
         args.add(String16("-a"));
@@ -64,6 +68,10 @@ int main(int argc, char* const argv[])
         }
     }
 
+    if (showListOnly) {
+        return 0;
+    }
+
     for (size_t i=0; i<N; i++) {
         sp<IBinder> service = sm->checkService(services[i]);
         if (service != NULL) {