Set default value of accessibility button mode

Set floating menu mode by default, but if the user uses accessibility button in the navigation bar to trigger their accessibility features before migration, then set it to keep the navigation button mode.

Bug: 173940304
Test: 1. Push old version of db and apk
      2. Enable/Disable accessibility features in navigation bar
      3. Push new version of db and apk
Change-Id: I465eb2190f0e1ca9a6a6be928752a07d5e9576da