SF: temporary disable FlagManagerTest.returnsOverrideFalse

Since we are left we a single server flag, we can't have a unique
flag for this test which is required since the flag implementation uses
static variables. We'll re-enable once we have a new server flag.

Change-Id: I49826eb0fbb76a389c063b6234b2a2131e358c75
Test: presubmit
Bug: 325334658