Log how much time running update_device.py took

Might be useful to quickly check how OTA optimization
(e.g. reducing time for otapreopt postinstall) are performing.

Bug: 190817237
Test: python3 system/update_engine/scripts/update_device.py --file out/dist/aosp_coral-ota-eng.ioffe.zip
Change-Id: I750fb959f1ed6d25b15c2ca05898e9415e276a8c
diff --git a/scripts/update_device.py b/scripts/update_device.py
index f672cda..165bc97 100755
--- a/scripts/update_device.py
+++ b/scripts/update_device.py
@@ -30,6 +30,7 @@
 import sys
 import struct
 import tempfile
+import time
 import threading
 import xml.etree.ElementTree
 import zipfile
@@ -441,6 +442,8 @@
   logging.basicConfig(
       level=logging.WARNING if args.no_verbose else logging.INFO)
 
+  start_time = time.perf_counter()
+
   dut = AdbHost(args.s)
 
   server_thread = None
@@ -548,6 +551,7 @@
     for cmd in finalize_cmds:
       dut.adb(cmd, 5)
 
+  logging.info('Update took %.3f seconds', (time.perf_counter() - start_time))
   return 0