Dont send RR events if not in foreground

Do not send refresh rate change callbacks to an app which is not in the
foreground. This will help in reducing the number of display change
callbacks.

Bug: 390107600
Test: Manually verified app in background is not getting any RR change callback
Test: atest DisplayManagerServiceTest
Flag: com.android.server.display.feature.flags.refresh_rate_event_for_foreground_apps
Change-Id: I5dba115a282e7d85f52a1b9870af058921ca44b4