Fix check when finding tasks to reuse.

- Since we added a new assistant activity type, we need to update the check
  for which stack to match the activity to.

Bug: 37526597
Test: android.server.cts.ActivityManagerAssistantStackTests
Test: #testLaunchIntoSameTask
Change-Id: I2d77ba8b026ee1ae692b026d20703fa56c061704