SystemUI: Disable alternate nav bar in car mode

Change ag/813926 introduced larger navigation buttons when in car
mode, and ag/891860 hid the recents button while car mode is active.
This change deprecates these behaviors, and puts them behind a flag.
A future change will remove this behavior entirely.

Bug: 31754884
Bug: 31754537
Change-Id: Iab7ec657105c08d8c45f76646ce0edbf3a5aa017