WifiManager: Restore network pin behavior on enableNetwork

This code is mostly a NO-OP for most apps & OEM's. The pinner only takes
effect if the app's target SDK < Android L. But, there might
be some OEM's who may want to enable this for all apps.

Bug: 129906426
Test: Compiles
Change-Id: Iccf67ba0d4ea8d9c5d3561a128908611636b420e