VisualVoicemailServiceTest: change asserts in setUp to assumes

Bugs are being generated because of assert failures on configs
where the test should be skipped. To avoid running the tests, the
setUp asserts are being swapped with assumes.

Flag: EXEMPT test only change
Fixes: 409100653
Test: VisualVoicemailServiceTest
Change-Id: Ib097f24355654fe7b835c1bfba32841f4ef4c3e3