Use getter within DesktopFirstRepository instead of field

The issue is that we previously copied an empty map from internal field, so each time we request this field we get an empty map

Bug: 441574781
Flag: com.android.systemui.shade_app_launch_animation_skip_in_desktop
Test: atest com.android.systemui.statusbar.phone.LegacyActivityStarterInternalImplTest com.android.systemui.animation.ActivityTransitionAnimatorTest com.android.systemui.statusbar.phone.StatusBarNotificationActivityStarterTest

Change-Id: I01f69eedd8fcc458a98e4ad5c6f702a0c2d630b2