NetworkStatsFactory: no double looping on stats + stacked interfaces

This patch uses the associative structure of the stacked/base interface
collection to avoid the two nested for loops over stacked/base
interfaces and all stats entries when correcting 464xlat traffic stats.

Consequently the list of stats entries is iterated only once.

Test: runtest -x frameworks/base/../NetworkStatsFactoryTest.java
Bug: 33681750
Change-Id: I84ae997fb693b909f431764697627b9957131732