Fix updateRecentTasksSplitPair unexpectly

While split pair switching, split remain as active so it might
record new bounds with old top task pair unexpectly.
Fix this by set flag to false on startTasksWithLegacyTransition
and reset to true after remote animation finished.

Fix: 213128767
Test: manual
Test: pass existing tests
Change-Id: I42b4c79558467dec23b789a3b5cc2f8f38cd1c36