Don't force nav bar shown if configured

There exists a configuration value to determine if the nav bar should
be shown when the keyboard is open. The InsetsPolicy should respect this
value. This value is currently used on automotive devices but doesn't
necessarily need to be restricted to a particular device type.

Bug: 229256359
Test: manual
Change-Id: I57ea71193476cb88bba8d406d039fe08e05a6b1c