Add User* OWNERS file (part 1)

Two parts are required due to b/175318571.

Test: N/A
Change-Id: I2bd2e1f71490e0d006ceeea177e2e5d5cd520a9a

MULTIUSER_OWNERS

+# OWNERS of Multiuser related files
+bookatz@google.com
+omakoto@google.com
+yamasani@google.com

services/core/java/com/android/server/pm/MULTIUSER_AND_ENTERPRISE_OWNERS

+# OWNERS of Multiuser related files related to Enterprise
+
+# TODO: include /MULTIUSER_OWNERS
+
+# Enterprise owners
+rubinxu@google.com
+sandness@google.com