Rename UITestDeviceHelper -> DeviceHelper.
Refactor DeviceHelper to be static.

This is of questionable importance, but it matches what we do with the
other helper classes.
nit: DeviceHelper variable naming conventions.
nit: Clarify DeviceHelper var names.
Use AndroidPlatformVersion enum for DeviceHelper version.
Add accessor methods to DeviceHelper.
nit: Add helper methods for DeviceHelper.init().
nit: Use more descriptive variable names in DeviceHelper.