Don't run testCantSetUserRestriction() on rooted devices.

This will also help in confirming if this is reason for the test
failures.

Bug: 31307295
Test: cts-tradefed run singleCommand cts-dev --module CtsMultiUserHostTestCases
Change-Id: I8f1f229c26955a3b27c6b3d68513a96b49d22afb
1 file changed