Proxy focused display ID from the sysui process to the screenshot process

Talking to FocusedDisplayRepository directly yields no actual data
outside of the main sysui process.

Bug: 399644508
Test: atest TakeScreenshotExecutorTest
Flag: com.android.systemui.screenshot_multidisplay_focus_change
Change-Id: Ice79cf3924699399e63a4c5ef2210c6518297db0