Cancel toasts when display mirroring starts

When display mirroring starts, all windows on the mirroring display must
be removed. Otherwise, SurfaceFlinger will not initiate mirroring.

Previously, if a toast was on the mirroring display, it prevented
mirroring from starting and remained visible.

This change updates DisplayPolicy to cancel toasts when mirroring
begins.

Bug: 389833251
Test: DisplayWindowSettingsTests
Test: NotificationManagerServiceTest
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Change-Id: If54418a0526b7b6c8f00e972bc0b39e362d6d7df