Fix captive portal app can be launched w/o MAINLINE_NETWORK_STACK permission

Caller should get SecurityException if called
ConnectivityManager#startCaptivePortalApp() w/o
MAINLINE_NETWORK_STACK permission. But now it will not get any
exception and can launch captive portal app successfully.

Bug: 132662433
Test: atest android.net.cts.ConnectivityManagerTest#testStartCaptivePortalApp
      w and w/o MAINLINE_NETWORK_STACK permission
Test: atest FrameworksNetTests NetworkStackTests

Change-Id: I61be1b0647f4165665a2d4166311adeac956e0d4