Move SafetyCenterMultiUsersTest back to post-submit.

These tests are too slow to pass the presubmit 60s deadline,
which makes them extra flaky. Hopefully the post-submit tests are
sufficient to catch regressions and for us to fix them quick enough.

Bug: 292825990
Bug: 292299392
Test: atest SafetyCenterFunctionalMultiUsersTestCases
Relnote: N/A
Change-Id: I156c8c4361a9f773de84b94373857b77f53f9075