Set RetailDemoModeService#mDeviceInDemoMode to false

Reset mDeviceInDemoMode when the device exits demo mode. This isn't a
supported configuration in the production devices, but it's used for CTS
in order to run tests back-to-back without reprovisioning the device.

Fixes: 34938521
Test: RetailDemoModeServiceTest
Change-Id: I5a7134815b2817e7b02b1a2df71932faee967e8d