Don't limit RetailDemoModeService to start only during demo mode.

- Update RetailDemoModeService to not do anything outside demo mode.
- Add am command get-started-user-state which is needed for cts tests.
- Update unit tests for RetailDemoModeService.

Bug: 31342350
Test: adb shell am instrument -e class com.android.server.retaildemo.RetailDemoModeServiceTest -w com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: Idf50512facd27d47d7111e75cbc2f7b260f49740