Added option to pre-create user templates to optimize first user creation time.

Initial user creation is slow because the system must prepare per-user data (like storage and
permissions) whose cost is proportional to the number of pre-installed apps. On automovive's
reference implementation, it can take more than 10s, which is a bad user experience.

This change lets OEMs pre-create some users , so that high initial-creation cost is "paid" during
the initial boot. On automotive, it improves the creation of an additional user (or guest user)
in about 7s (from ~17s to 9s).

Bug: 111451156
Bug: 132111956
Bug: 140750212
Bug: 140868593

Test: manual verification
Test: atest FrameworksServicesTests:UserControllerTest#testStartTemplateUser_background

Change-Id: I81de1b5376dc9c42b63be8853d7204c88826401f