Add unit test for MultipathPolicyTracker

This CL adds unit test to test onThresholdReached event
would unregister and re-register usage callback.

Ignore-AOSP-First: Urgent fix
Test: MultipathPolicyTrackerTest#testOnThresholdReached
Bug: 233733267
Change-Id: I6e43ae93055009d91b0795a4466f2c02f4403c0a
1 file changed