Stop VPN profiles by exiting VpnRunner instead of prepareInternal

Change to call the VpnRunner.exit to stop vpn profile
instead of using prepareInternal to leave the package intact.
This aligns with the way that VpnServices are disconnected
so that the package and other related information will not be
reset unexpectedly.

In current design, Vpn will examine if the current prepared
package matches to prevent an existing always-on VPN from being
dethroned by other apps when the VPN always-on is enabled.
However, when the VPN is disabled using stopVpnProfile, the
current package name will be updated to [LEGACY VPN]. The
current package will no longer be the same with the VPN package.
This resulted in the rejection of the VPN app to start the VPN
using VpnManager.startVpnProfile again.

Test: atest FrameworksNetTests
Test: manually test with VPN app to reconnect VPN when always-on
      is enabled.
Bug: 235322391
Change-Id: I83e1e1edf6c3a6653d87216afcd397f296f59cf2