Delay start of predictive back transition

This CL delays the start of the predictive back transition. With this CL, the back transition will only be started once the back gesture threshold was crossed. This prevents onStart of the previous activity being called whenever a tap happens within the back gesture area.

This may introduce a small delay of the animation start but testing suggests that the delay is acceptable. This will need to be further monitored and examined in the different flag rollout stages.

Bug: 301195601
Flag: com.android.systemui.predictive_back_delay_transition
Test: Manual, i.e. verified no regressions for back navigations with CL. Also verified manually that the introduced delay does not affect back animation responsiveness too much.
Test: BackAnimationControllerTest
Change-Id: I0255f4b34c7c9e9312bc503226fdb30dd3d1b9e6