Move UserManagerInternal out of android.os

Moving this class out of android.os will reportedly
greatly reduce ART overhead.

Test: compiles
Test: atest FrameworksServicesTests FrameworksCoreTests
Bug: 165817914
Exempt-From-Owner-Approval: no-op refactoring
Change-Id: I1f3e77120979e2f336e5f42e4ec6e459d6492083