Make DeviceState rely entirely on Nene.

Also exposes Nene UserReference through DeviceState, which makes some
tests easier.

This will mean all postsubmit (and soon presubmit) tests run on top of
Nene so we can shake out any lingering flakiness.

Test: atest NeneTest
Test: atest CtsDevicePolicyTestCases
Change-Id: Ib165f7e098207b2f6beb72d6e5b228ac9b8883df