Add flag for guarding StrictMode network restriction APIs

Add "strict_mode_restricted_network" as a new flag in the
"backstage_power" namespace. This will be used for guarding a new set of
StrictMode APIs for assisting developers in detecting otherwise
restricted background/service network access.

Bug: 317250784
Test: presubmit (flag is currently unused)
Change-Id: Idb1eacd602f1c1a26da38de54de0c5db0aa0cef3