Do not set different gravity for non-moveable nav bar

Some tests assume that if the bar cannot move, it should be always
at the bottom. Otherwise the tests may fail by Gravity.RIGHT on
a default landscape display.

Bug: 267792420
Test: adb shell wm size 2200x1800
      atest DisplayPolicyInsetsTests
Change-Id: Ieea801ceb8082349858a3fa8c4d324b6ad6278fb