[SP29.1] Simplify logic of calculating and applying data limit

This is a no-op refactoring that is needed for accommodating
follow-up change to send warning quota to hardware.

Test: atest NetworkPolicyManagerServiceTest NetworkStatsServiceTest
Bug: 149467454
Bug: 170699770
Bug: 170179169
Ignore-AOSP-First: avoid long automerger delay
Change-Id: I073934f3ac4d13b569ea3a295692c76451b7bbfa