adb: fix test_device.py shell protocol checks.

https://android-review.googlesource.com/#/c/210646/ added a
has_shell_protocol() function but the test_device.py tests were still
trying to use the now-missing SHELL_PROTOCOL_FEATURE constant.

This CL just switches test_device.py to correctly use the
has_shell_protocol() method.

Change-Id: Ie0a2f0dc07529843d25051a01e08fb677551a4e1
Test: `python -m unittest discover -v` on devices with and without
      the shell protocol.