[Android] Switch to DeviceUtils versions of GetMemoryUsageForPid and __str__.

build/android/tombstones.py

Index: build/android/tombstones.py
diff --git a/build/android/tombstones.py b/build/android/tombstones.py
index e7549ff73ed12bda06d3315929155a3c65d8a845..3c1624bab263567f10f0289d679fede824acf70d 100755
--- a/build/android/tombstones.py
+++ b/build/android/tombstones.py
@@ -150,7 +150,7 @@ def _GetTombstonesForDevice(device, options):
 
   device_now = _GetDeviceDateTime(device)
   for tombstone_file, tombstone_time in tombstones:
-    ret += [{'serial': device.old_interface.Adb().GetSerialNumber(),
+    ret += [{'serial': str(device),
              'device_now': device_now,
              'time': tombstone_time,
              'file': tombstone_file,