Fix unable to back when starting new TF on AE primary

The focsued app was not updated when starting an Activity C on
a new TaskFragment while the secondary container is pinned
(A|B) -> (C|B). That's because the focused app is used to be
updated to the top-most resumed activity of the top-most focused
Task.

In that case, the focused window is also null because both the
window of C and B are in a different TF than the focused
app's TF, so cannot be focused.

Updates the last resumed activity regardless of the activity
is the top-most one.

Bug: 329539209
Test: TaskFragmentTest
Change-Id: I7f307084b8814b922f7c9defd40bb468eddf71b9