init: fix writepid wrong pid written in file(s)

When child (pid==0) executes fork branch 'pid_str'
will always be 0 writting wrong value to file(s).
Fix by utilizing getpid() to obtain correct pid.

Tracker link provides some insight on why use cases
aren't broken.

Bug: https://code.google.com/p/android/issues/detail?id=200392

Change-Id: I29989365a8bac94502c090918fa30e5cc88eb1ea
Signed-off-by: Anestis Bechtsoudis <anestis@census-labs.com>

init/service.cpp

@@ -416,7 +416,7 @@ bool Service::Start(const std::vector<std::string>& dynamic_args) {
             }
         }
 
-        std::string pid_str = StringPrintf("%d", pid);
+        std::string pid_str = StringPrintf("%d", getpid());
         for (const auto& file : writepid_files_) {
             if (!WriteStringToFile(pid_str, file)) {
                 ERROR("couldn't write %s to %s: %s\n",