adb/test_device.py: fix finding md5 command on pre-L platforms

606835ae5c4b9519009cdff8b1c33169cff32cb1 in platform/system/core made
adb.device.shell() throw adb.ShellError instead of
subprocess.CalledProcessError.

Change-Id: I7f579887a9aab8d11cbb2c419b8406f51d6b1bd6
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>

python-packages/adb/test_device.py

@@ -383,7 +383,7 @@ def get_md5_prog(device):
     try:
         device.shell(['md5sum', '/proc/uptime'])
         return 'md5sum'
-    except subprocess.CalledProcessError:
+    except adb.ShellError:
         return 'md5'