Add feature flag for providers to support backup&restore networktypes

Adds feature flag for network
Support backup & restore for allowed_network_types_for_reasons. This
feture supports only enable_2g reason.

Bug: 314734614
Test: atest com.android.providers.telephony.TelephonyProviderTest
Change-Id: Ic47b761bbbd78eeeda7d1a5d0de702b324889cc9