isForegroundUserAdmin public API

Introduce an API by which callers can determine whether the foreground
user is an admin (even though the caller may not know who the foreground
user is). Foreground is synonymous with current.

Also adds an additional Flag for future communal development, where this
API is expected to be useful.

Test: atest android.multiuser.cts.UserManagerTest#testIsForegroundUserAdminUser_withAdditionalUser
Test: atest android.multiuser.cts.UserManagerTest#testIsForegroundUserAdminUser_admin
Change-Id: I2f2d74681b302a64991b11b32382558118ba798b