Add aconfig_declarations for flags in android_core_networking

Add aconfig_declarations so that platform code can use this flag.
Following CLs(aosp/2606670) use this flag to sync the mainline behavior
change and platform behavior change.
Having merged-in to avoid being merged to udc-mainline-prod where
aconfig soong targets are not supported

Bug: 267870186
Test: TH
Merged-In: I4870fbe882ce0ec995dcaeb8a2624071fee6dce7
Change-Id: I2260dc3d60689da9328fe4f98c3f3048d96bbf6d