[2/n] Add users repository for the multiuser widget

This change adds a repository that reads users info and stores it into a
local database.

Bug: b/375584159
Test: atest MultiuserWidgetUnitTests
Flag: android.multiuser.multiuser_widget
Change-Id: I01cdcc873aef4cbf4a0d80ff9a5ed62f7b0ec11e
7 files changed