Start back gesture tracking in BAC on ACTION_DOWN

Since startBackNavigation is no longer called in onGestureStarted, we
don't have to delay its invocation anymore.

Bug: 301195601
Test: Manual, i.e. verified that predictive back system animations work
      as expected.
Test: BackAnimationControllerTest
Flag: com.android.systemui.predictive_back_delay_wm_transition
Change-Id: I72a7993d596d41bd499bd299f92498e55b0a2e10