Add a DeviceConfig flag that will be used to control whether callbacks
should be sent selectively based on whether the receiving method has
been overridden (will be implemented in a future change).
This adds utilities to allow ConnectivityManager to check whether a
ConnectivityService flag is enabled, as that feature will work across
ConnectivityManager and ConnectivityService. ConnectivityManager runs in
app process and does not necessarily have permissions to use
DeviceConfig. Also if it checked DeviceConfig separately from
ConnectivityService, values may be out-of-sync.
Test: atest, and see change using this
Change-Id: I204fa719cc627e84edc5c1e043d947f5ef3f8dee
aa355ac875