Refactor startIntentAndTaskWithLegacyTransition

Param intentFirst is not necessary because we could use stagePosition
to ensure intent side is left or right.
Remove it and ensure start task on main stage at split side.

Bug: 218613028
Test: pass existing tests
Test: manual
Change-Id: I3b8fc74b18c4d48fe73b501be696c3d59677183b