Add flag support for launching into guest by default

Adds support for a build flag that allows the system to use either the
current reference model logic or to boot into guest as the default user.

Bug: 122543530

Test: Build target with flag on and run:
adb shell dumpsys user | grep "All guests ephemeral: false"
Reboot HU and note that guest is still running

Test: atest CarServicesTest

Change-Id: Ic0aadbb4bbb4fe0cb8c1b3b4380c56b01f442ec0
(cherry picked from commit 2807273cee6b14c236dd76560b02284e65630f7e)