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: Ib70fe6fad107f3e9dce9ce673188c5ce5dc1ad7b
Merged-In: I1025da29beb53259f57bd9ca5648b32f2847ed4a
Merged-In: Ib70fe6fad107f3e9dce9ce673188c5ce5dc1ad7b
(cherry picked from commit 72b3ab18)