Switch fastboot/init/libprocessgroup to std::this_thread::sleep_for.

Bug: http://b/32878766
Test: boots
Change-Id: Ie0ddfb7e60f2da5f6eefbb10c83a92e88c137ae3
diff --git a/libprocessgroup/processgroup.cpp b/libprocessgroup/processgroup.cpp
index 7c15429..eb66727 100644
--- a/libprocessgroup/processgroup.cpp
+++ b/libprocessgroup/processgroup.cpp
@@ -31,12 +31,15 @@
 #include <chrono>
 #include <memory>
 #include <mutex>
+#include <thread>
 
 #include <android-base/logging.h>
 #include <private/android_filesystem_config.h>
 
 #include <processgroup/processgroup.h>
 
+using namespace std::chrono_literals;
+
 // Uncomment line below use memory cgroups for keeping track of (forked) PIDs
 // #define USE_MEMCG 1
 
@@ -288,7 +291,7 @@
     while ((processes = killProcessGroupOnce(uid, initialPid, signal)) > 0) {
         LOG(VERBOSE) << "killed " << processes << " processes for processgroup " << initialPid;
         if (retry > 0) {
-            usleep(5 * 1000); // 5ms
+            std::this_thread::sleep_for(5ms);
             --retry;
         } else {
             LOG(ERROR) << "failed to kill " << processes << " processes for processgroup "