2/ Clear all recents tasks before running legacysplitscreen tests

Sometimes previous launched activity will affect the order of
current test activity in recents and cause the flicker test flaky.
Trigger openQuickStepAndClearRecentAppsFromOverview() in setup()
to ensure overview/recents is clear before the tests

Bug: 175728979
Test: atest com.android.wm.shell.flicker.legacysplitscreen
Change-Id: I6a6aae362617bb7948473aaafaab8f2c3eec0bc1