Check task switch by comparing the previous top task

Previously, the top task used for checking task switch is
obtained after reusing existing task. So the existing target
task may have been moved to front, that causes misleading
that there is no task switch, and no starting window.

This change gets top task at the beginning of the start request,
so it can get the actual previous task.

This also makes commit 60f9c97a effective again for the common
cases to back-transfer the starting window.

Also remove unused mKeepCurTransition.

Bug: 195007725
Test: ActivityStarterTests
Test: There should be a starting window when launching new activity
      on an existing task from different task.

Change-Id: Ic402ec83be47e6a5e12eee2c700f9d8079e89389