Setup aconfig for desktop_users_and_accounts

This change introduces an aconfig file for desktop users and account
features and adds a first flag which adds a sign out option to the user
switcher.

Test: manual - adb shell aflags list
Flag: com.android.systemui.user_switcher_add_sign_out_option
Bug: 381478261
Change-Id: Ie88daca4828e67a81f34daa3e1eab5d32d2a12e7