Fix MixedDeviceOwnerTest test failure

NullPointerException was thrown while running the test because
the top activity of the most recent task is null. This is a
regression caused by commit 1ff574d7.

It happens when adding a recent task because of resuming the next
activity while the top-most activity finishes (the activities are
in two different Tasks). The finishing task still has the higher
z-ordering in the WM hierarchy which therefore was remained be the
most recent task.

The finishing task should be excluded and the most recent task
should be the one that is resumed.

Bug: 326367294
Test: atest RecentTasksTest
Test: atest MixedDeviceOwnerTest
Change-Id: Iba8a2004a58a4d440a0c62c59ae89b5e4ca1253c