Use UiObject2.fling API for recents jank test

UiObject2's fling API enables flings to be performed
with a specific velocity value. Use that API here
for stable jank benchmarks and to allow different VelocityTracker
strategy for computing velocity without affecting the
benchmark results.

Test: bit
UbSystemUiJankTests:android.platform.systemui.tests.jank.SystemUiJankTests
Tests had to be run after initial device setup as described in
go/run-jank
Bug: 64680775
Bug: 35412046

Change-Id: Ieb57bd55a46252917bc89eb4f871e8379cb16051