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

Bug: http://b/32878766
Test: boots
Change-Id: Ie0ddfb7e60f2da5f6eefbb10c83a92e88c137ae3
diff --git a/init/devices.cpp b/init/devices.cpp
index d422ba7..5098fb3 100644
--- a/init/devices.cpp
+++ b/init/devices.cpp
@@ -35,6 +35,7 @@
 #include <linux/netlink.h>
 
 #include <memory>
+#include <thread>
 
 #include <selinux/selinux.h>
 #include <selinux/label.h>
@@ -841,7 +842,7 @@
     if (booting) {
         // If we're not fully booted, we may be missing
         // filesystems needed for firmware, wait and retry.
-        usleep(100000);
+        std::this_thread::sleep_for(100ms);
         booting = is_booting();
         goto try_loading_again;
     }