[Safety Labels] Make multiuser tests more reliable

We are noticing test failures on slower devices. To alleviate that:
* increase waiting time as slow devices don't open the activity in time
  for the assertions
* remove some unnecessary assertions which are covered in single user
  variant
* add exclude annotations for tests so that tests which will have
  assumptions failed don't run at all
* include selector information in assertion message

Bug: 281550547
Test: atest CtsPermissionMultiUserTestCases
Change-Id: Ie77d8c8782fde02226c5f9ff07c9f4ca0551c68e