Restore the taskAffinity behavior of singleInstance Activity

The unique taskAffinity is reasonable for the task resolving policy.
However, it breaks the Recent Task behavior where the same taskAffinity
Task should be removed from Recents while adding the singleInstance
activity task to the Recents.

Bug: 288627637
Test: atest RecentTasksTest
Change-Id: Ib68e9d4abe02cd94f46045ed0a9cea4212f24e3d