Use -s to pass string date to 'adb shell date' command to set date in provision.

The 'date -u' was not working on N5 devices. The -s option seems to work across
devices. I've tested on N4 and N5.

BUG=406848

Review URL: https://codereview.chromium.org/668013003

Cr-Commit-Position: refs/heads/master@{#300542}

build/android/provision_devices.py

@@ -190,7 +190,8 @@ def ProvisionDevice(device, options, is_perf):
                      battery_info.get('level', 0))
         time.sleep(60)
         battery_info = device.old_interface.GetBatteryInfo()
-    device.RunShellCommand('date -u %f' % time.time(), as_root=True)
+    device.RunShellCommand('date -s %s' % time.strftime('%Y%m%d.%l%M%S'),
+                           as_root=True)
     # TODO(jbudorick): Tune the timeout per OS version.
     device.Reboot(True, timeout=600, retries=0)
     props = device.RunShellCommand('getprop')