test: sensor_manager: add test and sensor_mock

Adds a mock sensor for use in future tests, as well as
adds a unit-test to sensor_manager_unittest to check that the
type of a sensor added is not validated against anything.

Change-Id: I684ec71c02f427cfa7a414a6a4162beaba2269da
Signed-off-by: Patrick Venture <venture@google.com>
2 files changed