PrivacyDotWindowController: dynamically add/remove windows as needed

Before, the all 4 windows (one for each corner) were always added on
startup. Since having these windows is costly, we optimize to show
them when needed.

This only affects the connected displays implementation of the privacy
dot.

Flag: com.android.systemui.shared.status_bar_connected_displays
Fixes: 422343339
Test: PrivacyDotWindowControllerTest
Change-Id: I38a6af5a9027f68a521b9b0b84cdb378d63fd406