Use DesktopExperienceFlags to call Flags.screenshotMultidisplayFocusChange

This is a requirement to be able to use that feature as part of the 25Q3 developer option.

Fix: 413735060
Flag: EXEMPT (flag infra)
Test: Build and presubmits
Change-Id: Ib3a8a27770c410b35c5579a71f71de54b6c5665b