add universal_newlines=True in KillADB()

This fixes https://console.cloud.google.com/errors/CLH79JmVktryMg?time=P1D&project=chromium-swarm-dev

Bug: 1198820
Change-Id: Id4041a93f17ef5d513b5e7587d051883d37b6fef
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/python-adb/+/2936244
Auto-Submit: Junji Watanabe <jwata@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
diff --git a/adb/contrib/adb_commands_safe.py b/adb/contrib/adb_commands_safe.py
index b8791fc..aa3bab2 100644
--- a/adb/contrib/adb_commands_safe.py
+++ b/adb/contrib/adb_commands_safe.py
@@ -75,7 +75,9 @@
     time.sleep(0.001)
 
   try:
-    processes = subprocess.check_output(['ps', 'aux']).splitlines()
+    processes = subprocess.check_output(
+        ['ps', 'aux'],
+        universal_newlines=True).splitlines()
   except subprocess.CalledProcessError:
     _LOG.error('KillADB(): unable to scan process list.')
     processes = []