Only migrate 2-button setting if set

- We have only shipped configurations with:
  default=3 button + setting to opt in
  default=2 button + no setting to opt out

  So to prevent migrating in the case where a user has restored from
  a device (1) to a device (2) where the setting may be false (but
  previously showed 2 button nav), we only migrate the nav bar mode
  to the 2 button overlay whenever the setting is set. This means
  that the user will fallback to the device default nav bar mode
  which corresponds to the previous default modes for each device.
- Also remove temporary nav bar mode migration for dogfood users

Bug: 128548249
Test: Migrate from P to Q with swipe up setting set to false

Change-Id: I7218eeb13fec50b2fe3e89f75ef0d2d4ab67aff1