Remove some assertions to speed a test

Improves speed of
moreIssuesCard_twoIssuesAlreadyShown_expandAdditionalIssueCards which on
some very slow devices is struggling, by splitting out the assertions
about setup from the main behavior under test, and covering across two
test cases.

Test: atest SafetyCenterActivityTest
Bug: 293388777
Relnote: Simplify an MTS test for speed purposes
Change-Id: I6a884d419f0af932e8c1214a8f4291de6146c76e
Merged-In: I6a884d419f0af932e8c1214a8f4291de6146c76e