Update DisplayWindowSettings.mShouldShowSystemDecors in setCurrentUser()

During the boot phase, the call to WindowManagerService#setCurrentUser()
triggers DisplayWindowSettingsProvider.setOverrideSettingsForUser(),
which resets all SettingsEntry objects of DisplayWindowSettings to null,
including mShouldShowSystemDecorations.

Because mShouldShowSystemDecorations is set upon onDisplayAdded() and
setCurrentUser() happens afterwards, shouldShowSystemDecorsLocked()
will consistently return false, regardless of the correct value.
This incorrect persists until the next onDisplayChanged() call updates
DisplayWindowSettings.mShouldShowSystemDecors.

This results in the issue where home activity and wallpaper are not
shown on the overlay display after a reboot if the overlay display was
added before the reboot.

This CL fixed this issue by updating
DisplayWindowSettings.mShouldShowSystemDecos after
setOverrideSettingsForUser() in setCurrentUser().

Bug: 409887951
Test: manually
Test: run go/cd-smoke
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Change-Id: I0e9947df10d70c3a47b943226defefbbc26aa9b3