Adding a warning dialog before restarting demo session

Instead of immediately restarting the demo after user inactivity timeout,
now the system shows a warning dialog that counts down for 6 seconds,
during which the user can choose to continue the current session or
restart the session immediately.

Bug: 29520255
Change-Id: I060f700dcbbb6f64ae5fe95e3f3027bc3c33b38d