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: I3a667cfa14d51e481bb4e99301c10319d2600e00
Merged-In: I44c111577643bacb35b532cb156a61d30cfea31a