Settings: Clean up WifiManager APIs

Promoting WifiManager#registerSoftApCallback() and
WifiManager#registerNetworkRequestMatchCallback()
to @SystemApi.

Changed method signatures to conform to API guidelines.

Also removed dead code: WifiManager#getWifiState()
never returns WifiManager#ERROR.
Remove reference to it, since it is an @hide API.

Bug: 143970861
Test: compiles
Change-Id: If4294d106f0acb5c67b5eab1f6f99bc52efc10f2