Update swipe edge of BackTouchTracker in onMove

When performing a back gesture with the trackpad, it's not clear from
the beginning, which edge the gesture is performed on. Therefore, this
CL updates the swipe edge in BackTouchTracker in onMove, such that
clients of BackAnimation can update the swipe edge after the gesture
tracking has already started.

Bug: 301195601
Test: Manual, i.e. verified that predictive back animations animate as
      expected when using the trackpad back gesture
Flag: com.android.systemui.predictive_back_delay_wm_transition
Change-Id: I2e1fbddef8280c91b7d996b26726bd41a7d13a5b