Fix remaining to-do items of building TaskFragment

- Remove computeConfigurationAfterMultiWindowModeChange because it is
  no-op for couple of years and none blames at that.
- set minWidth & minHeight for TaskFragment when TaskFragment is added
  to the window hierarchy.
- Remove the logic to cleanUpActivityReferences to the parents since
  mPausedActivity and mResumeActivity are tracked to leaf TaskFragement
- Move clear mLastPausedActivity logic from root Task to TaskFragment

Bug: 189384393
Test: presubmit

Change-Id: I0156d9c709ddffe11389e2bd0ae9ad6b96f707be