Use DisplayManager to retrieve displays to screenshot

DisplayRepository doesn't seem to update its cache correctly when the
list of available displays changes (possibly due to screenshots being in
a separate process). Switching to Display Manager fixes the issue,
ensuring that we get the correct focused display in multiwindow mode.

Bug: 423959791
Test: manual (using simulated external displays),
TakeScreenshotExecutorTest
Flag: com.android.systemui.screenshot_multidisplay_focus_change

Change-Id: Ic4c2f6aa214b1176408dfdf437cbafcf838779ff