Stabilize and speedup RecentTasksTest

NPE happens due to race condition when setting new display.
Move the creation of display to where the test needs it,
that reduce 20% (~20s) test time.

Bug: 140284973
Test: atest RecentTasksTest
Change-Id: I84f9a1ec270395d8d7d03880ec8a5370b6ae2eb0