Refactor startIntentAndTaskWithLegacyTransition

Migrate startIntentAndTaskWithLegacyTransition and
startTasksWithLegacyTransition as single function and remove
unnecessary param intentFirst because we could use sidePosition to
decide stage position.

Bug: 218613028
Test: pass existing tests
Test: manual
Change-Id: I1067ebb1ef2b101c369452728e4e626f8513e579