Add @RequiresApi annotation for sdk version S for UserManagerState.

The interface is not used for sdk version R because call to
ActivityManager.getCurrentUser() is not supported. Explicit checks for
minimum sdk version had to be added to all the places that were using
either UserManagerState or TestUserManagerState instances. This had to
be done to satisfy presbumit lint checks.

Bug: 329508093
Test: atest DocumentsUIUnitTests
Change-Id: I32317805d1aae3b0a0fb89a486a41c3b24889bc4