Dismiss login screen before starting tests

This CL adds a command to dismiss the login screen before starting the
tests.
On desktop devices, prior tests may leave the login screen active so we
want to ensure it’s dismissed.

Flag: EXEMPT - test fix
Bug: 416610249
Test: ConnectedDisplayCujSmokeTests

Change-Id: I9e66a18e8a392da065fa790397316cc5ad479449