Add test for Vpn#prepare()

Add test for Vpn#prepare() to check if the SecurityException will
be thrown when the caller is not the SYSTEM_UID and doesn't have
CONTROL_VPN permission and its package doesn't match the oldPackage
or the newPackage.

Ignore-AOSP-First: It's a part of security patches.
Bug: 191276656
Test: atest FrameworksNetTests:VpnTest
Change-Id: Ibe0b245561b00442a6377fb849603f8b9026e9c4
Merged-In: Ibe0b245561b00442a6377fb849603f8b9026e9c4
1 file changed