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.

Test: MultipathPolicyTrackerTest#testOnThresholdReached
Bug: 233733267
Fix: 145963406
Change-Id: Icee43a4ee913a6459a47632f608fa6cb8b2ed4c2
Merged-In: Icee43a4ee913a6459a47632f608fa6cb8b2ed4c2
  (pure cherry-picked from ag/18583185)