Notify TaskBar via onDisplayRemoveSystemDecorations when starting mirroring

In this CL, when taskbar receives onDisplayRemoveSystemDecorations, it
performs similar actions to those triggered by onDisplayRemoved.

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

Change-Id: Id5caffcf39de7f080eeb0534c223c3fcd8341b4a