Add Unit Tests to Check Default Strategy Setup

VelocityTracker uses different default strategies for different axes,
in cases where it's not supplied with an explicit strategy. This CL
tests that behavior for all the axes currently supported (X,Y, and
SCROLL).

Bug: 249171348
Test: unit tests
Change-Id: Iba9dcc0a7dee39d30d10b60f1058c3657be09ad7