Fix crash when incrementing data usage request

This fixed a request leak problem which was introduced in previous
Android release but caught recently, where the budget of
MultipathPolicyTracker was cleaned before unregistering requests,
the logic in the tracker would skip the unregister if there is
no budget.

Ignore-AOSP-First: Urgent fix
Test: MultipathPolicyTrackerTest#testOnThresholdReached
Bug: 233733267
Fix: 145963406
Change-Id: Icee43a4ee913a6459a47632f608fa6cb8b2ed4c2