Move Setting constants for NetworkStack

From mainline perspective, we should use android flag api
instead of using Settings. Thus, move the definitions into
NetworkStack and apply new flag design.

Bug: 123167629
Test: atest NetworkStackTests
Change-Id: I9602263f0bff5d8e942bd652de69ccfcb3034a2f
Merged-In: I6820300c412f94989a5fce7bd6c6f2a3b983b96e
Merged-In: I4f6d130ffbee14f5087d75a8bc211680a34be682
(cherry picked from commit 01440ea9)