Integration of Storage API to flutter - Users, Units

Integration of Storage API to flutter - Users, Units

Updated user and units notifiers to make use of persistent storage. New
users are stored with a name and id in the default namespace, and their
unit preferences (distance, temperature, tire pressure) are stored in
namespaces corresponding to each user's id.

Added new initialize_settings to load user and unit settings from
storage on startup. For first-time startup, default users with ids have
been added to storage as well.

Added unit tests for new user and unit handling.

Bug-AGL: [SPEC-5228]
Change-Id: I9cbcfc3ea5045dcb27db1b05e332f89abfc284a1
Signed-off-by: Tom Kronsbein <tom.kronsbein@d-fine.com>
Signed-off-by: Ludwig Schwiedrzik <ludwig.schwiedrzik@d-fine.com>