storaged: remove usage of String16::std_string

<string> is being removed from String16

Test: adb shell dumpsys storaged <all options>
Change-Id: I234ecd64d9c481cb2eeee883a715f2fbfaecc511
diff --git a/storaged/storaged_service.cpp b/storaged/storaged_service.cpp
index a335298..f968ed7 100644
--- a/storaged/storaged_service.cpp
+++ b/storaged/storaged_service.cpp
@@ -97,19 +97,19 @@
         if (arg == String16("--hours")) {
             if (++i >= args.size())
                 break;
-            hours = stod(String16::std_string(args[i]));
+            hours = std::stod(String8(args[i]).string());
             continue;
         }
         if (arg == String16("--time_window")) {
             if (++i >= args.size())
                 break;
-            time_window = stoi(String16::std_string(args[i]));
+            time_window = std::stoi(String8(args[i]).string());
             continue;
         }
         if (arg == String16("--threshold")) {
             if (++i >= args.size())
                 break;
-            threshold = stoll(String16::std_string(args[i]));
+            threshold = std::stoll(String8(args[i]).string());
             continue;
         }
         if (arg == String16("--force")) {