backnav: don't reset back tracker at start

- updateStartLocation will reset the BackTracker internal state and
  update the start position of the progress calculation
- Updating at animation start will cause a significant offset for small
  screen devices. This is especially obvious on Wear OS.
- Skipping will not cause a direct jump in animation. There is a visible
  spring effect when the animation catches up with back progress.

Bug: 381150378
Test: build & flash
Flag: EXEMPT, bug fix
Change-Id: I2fb1be277f328fc3ef556804c2bc6dbc02c68c29