Notify wallpaper via onDisplayRemoveSystemDecorations when starting mirroring

This in CL, when the connected display is switched from extended mode to
mirroring mode:
(1) WM sends onDisplayRemoveSystemDecorations() to wallpaper.
(2) Wallpaper then performs similar actions to those triggered by
onDisplayRemoved().

Bug: 352461502
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Test: atest WallpaperManagerServiceTests
Test: manually (adb shell settings put secure mirror_built_in_display {1|0})
Change-Id: I4482bf6b609da7a6ab5fb44edbe6734ff766700e