Use flags.aconfig in platform for android networking

flags.aconfig for android networking is under
packages/modules/Connectivity.
But, currently, mainline code should not read trunk stable flag.
To avoid confusion, this CL moves flags.aconfig to platform.

Bug: 315302281
Test: TH
Change-Id: I49db6f8c02b50a32f851678114694a029c797042
Merged-In: I7142d937a59a880b68c11fff9254c84b39ce072f