[WIFI] Protect broadcasts used by the Wi-Fi framework

Make sure the broadcasts used by the Wi-Fi framework are protected:
i.e. can only be transmitted by privileged components (as opposed to
any 3rd party app).

Broadcast: Suggestion API

Bug: 142001564
Test: atest android.net.wifi
Test: atest com.android.server.wifi
Change-Id: I1b926ddcc0878be37c573c697fa5498715221413