Fix issue with non-focusable PiP activities being resumed.

- When the change to finish activities was made in ag/2067154, it exposed
  an issue in pinned stacks where we would resume the next top activity
  once the top activity finished pausing. Normally, the pinned stack is
  not focusable, but since it has an alwaysFocusable menu activity it
  becomes the focused stack and falls into this case.

  Instead of finding the next top activity, we need to find the next top
  focusable activity to resume, and fall through to focusing the top
  activity in the next focusable stack if there is none.

Bug: 37199067
Test: android.server.cts.ActivityManagerPinnedStackTests
Test: #testNoResumeAfterTaskOverlayFinishes
Change-Id: Ib79826ff38bb3beb38a40183ddc6819e5040bb27