Update snapshotStartingWindowLayerCoversExactlyOnApp to only check snapshot starting windows overlap app when app exists

Test: atest com.android.server.wm.flicker.ime.ShowImeOnAppStartWhenLaunchingAppFromFixedOrientationTest

Bug: 293096776
Change-Id: I7be721d482c79bb18677f04bc66b972cda5b8ef3
2 files changed