Remove setting of reuse task on an ActivityRecord.

Remove setting of reuse task on activity record.

Setting the reuse task can lead to confusing states.
It would be possible to have activityRecord.getDisplayArea()
to differ from activityRecord.getTask().getDisplayArea().
Removing this possibility makes it easier to reason about the
code.

Bug: 153920825
Test: atest WmTests:ActivityStarterTests
      atest WmTests:RootActivityContainerTests
Change-Id: Ida68d1c0e995ef816191d4e17335bd07ec392cc7