Turn AppsRowManagerTest's config check into a @Before function

It seems like the test runner doesn't like assumption failures in
@BeforeClass functions, so change it to just a @Before function.

Bug: 429955274
Test: atest 'DocumentsUIGoogleTests:com.android.documentsui.dirlist.AppsRowManagerTest'
Flag: TEST_ONLY
Change-Id: Ie5363e3ba685b1c8cf68ee9d41112a06b6be38e8