Fixes missing FLAG_ACTIVIY_NEW_TASK in startActivity

This fixes a fast-failure in 3 of 4 of the tests, others
remain flaky or broken but progress further.

Bug: 119893767
Bug: 124058588
Test: atest WmTests:TaskStackChangedListenerTest
Change-Id: Ifb548a30dc23de04380911d2956adc581c182a96