Enable secondary_user_on_secondary_display for ActivityStarterTests

secondary_user_on_secondary_display is for background users that have
access to UI on assigned displays (a.k.a. visible background users) on devices
that have config_multiuserVisibleBackgroundUsers enabled. The main use
case is Automotive's multi-display Whole Cabin experience where
passengers (modeled as visible background users) can interact with the display
in front of them concurrently with the driver (modeled as the the
current user) interacting with driver's display.

Fixes include
- Instead of asserting on default display, assert on the main display
  assigned to the user.

Flag: NONE (bugfix)
Bug: 350742111
Test: atest CtsWindowManagerDeviceActivity:ActivityStarterTests
Test: atest --user-type secondary_user_on_secondary_display ActivityStarterTests
Test: atest --user-type secondary_user ActivityStarterTests
Change-Id: I1e5a9c33a16f1be5cc84aa1d93c088190ca7039e
(cherry picked from commit f9b82d89295c03d0c8e2e17d21fc73a34f251c7b)