[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).

Bug: 146642727
Test: atest android.net.wifi
Test: atest com.android.server.wifi
Change-Id: I9a0bf4428774b13cc6414a223f378e09341a6a55