Add configuration parameters to control user switcher behavior

Adds two new configuration parameters. One is used to control
automatic enabling of user switcher setting upon user creation,
and second one controls if users (e.g. owner/admin) are allowed
to manually change the user switcher setting though UI.

Bug: 375394047
Test: atest UserManagerServiceTest
Test: create user, check value of user_switcher_enabled via ADB
Flag: EXEMPT Desktop-only change
Change-Id: Iafcadcb5bed5dd924402cd54a6d468aaa78f8cfd