Move the setup to the actual test body

For some reason it looks like there's a race condition where shutting
tasks down they are sometimes still there and the next Intent is not
delivered correctly. Move the start activity into the test body and
ensure that the tasks are properly killed in the setUp method.

Bug: 420176278
Test: atest com.android.documentsui.TrampolineActivityTest
Flag: com.android.documentsui.flags.redirect_get_content_ro
Change-Id: I606b4be35492e0a77e6c97c6bdf514ed80cbcde8