[DLC vNext] Use synchronized methods in DeviceStateController for thread safety

Also added unit test for testing the atomic property of
setNextStateForEvent.

Bug: b/286324034

Test: m RunDeviceLockControllerRoboTests

Test: manually tested normal provisioing flow and deferred provisiong
flow, worked as intended except an already existing issue as stated in
b/286324034#9. The exising issue isn't caused or affected by this CL.

Change-Id: Ib5c79fcaca979be0cdee80735fa37d7cd4ea36ba