Prevent ArrayIndexOutOfBoundsException for some invalid sysui_nav_bar values

A user can change sysui_nav_bar via adb.

If value of sysui_nav_bar doesn't contain two semicolons, split String array
is accessed out of bounds throwing an exception, sending SystemUI
in an exception loop.

Test: manual
Test: adb exec-out settings put secure sysui_nav_bar "home;back"
Change-Id: Ia9d74be36d287085650393476029489c9a359a0f