Add @SystemApi allowAutojoin(int, boolean) to WifiManager

WifiManager.allowAutojoin(int, boolean) lets a user toggle whether a
saved network should be allowed to be auto-joined. This "allowAutojoin"
bit is stored within the WifiConfiguration of a saved network.

Bug: 139199957
Test: atest FrameworksWifiApiTests
Change-Id: Ia8650abb59dc53a010accc44bd7b6e28facce992