[iwlan] config for connection persistence support

Not all implementation of Iwlan data service can support
persistence of iwlan tunnel across service crash/restart

A new config, config_wlan_data_service_conn_persistence_on_restart,
is added to handle this case.
This new config allows data service to declare support for ipsec
tunnel persistence across service restart. Telephony framework can
take action to clean up dangling connections in case service restarts
and if this flag is set to 'false'

Default value is 'true' [i.e no change from existing behavior]

Bug: 158043105
Tests: on device test involving killing/restart of data service
Change-Id: If328b6e1467095e059e532f55e0062211b7c80c9