Adding RetailModeService to handle retail mode

The service listens for the global setting DEVICE_DEMO_MODE and switches
device to demo mode when it is changed to 1. Also, acquires the wakelock
to
keep the screen on and puts up a notification to wipe and reset the demo
session when in demo mode.

Bug: 27280140
Change-Id: If1843016889ec2c50818c67432d60f33063e0986