Add support for app data accounting for in-kernel dataplanes

This change ensures that app data accounting works correctly within the
confines of in-kernel dataplanes, as used by platform VPNs and the VCN.

Notably, the VCN MUST NOT specify the IMSI, as that would lead to double
counting of the interface statistics.

Bug: 175853498
Bug: 190620024
Test: atest NetworkStatsTest FrameworksVcnTests
Original-Change: https://android-review.googlesource.com/1749070
Merged-In: I768907cd3dd2028c7040cddd81fc71a5ce69bbdb
Change-Id: I768907cd3dd2028c7040cddd81fc71a5ce69bbdb