Refactored device.py and tests/device_unit_test.py.

Removed the unused abstract Device class from device.py
and replaced mocking of AdbDevice method calls with
mocking of subprocess.run calls in the
test_root_device_success and
test_root_device_times_out_error unit tests
of tests/device_unit_test.py.

Test: atest device_unit_test
Bug: 355053544

Change-Id: I57e33810f247d6c9d30906c0fdc672e4695d3378
2 files changed