Update shouldShowSystemDecors on Display.canHostTasks changes

When the "Mirror Built-in Display" switch is toggles,
Display.canHostTasks changes, and WM is notified via onDisplayChanged().

In this CL, DisplayWindowSettings.mShouldShowSystemDecors is updated
based on the value of Display.canHostTasks:

If true, system decorations is started in the display;
If false, all tasks on the display are removed or reparented to the
defualt display.

Bug: 352461502
Bug: 374240163
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})
Test: DisplayContentTests

Change-Id: I8dfb0f7e96386d285fd7e40d76602dab95c98187