[Contextual Edu] Show dialog after back gesture completed

Show dialog after back swipe gesture is completed to avoid interrupting the gesture

Bug: 369991056
Test: Manual, open an app and perform back swipe twice. The back gesture is successful with dialog displayed
Flag: com.android.systemui.keyboard_touchpad_contextual_education
Change-Id: I18e2e079484e41b0dcde886b1f164473596d4ac6