Have a method to stop VPN and send event to VPN app

Have a method to stop VPN and send
CATEGORY_EVENT_DEACTIVATED_BY_USER event to VPN app.

Both of prepareInternal() and stopVpnProfile() have the same
behavior, so have a method for them to reduce the duplicate.

Bug: 236409954
Test: atest FrameworksNetTests:VpnTest
Change-Id: I54b550e6f972a13f61cbcf15ee1fb9fe3c313d7b
(cherry picked from commit 395c0909)
Merged-In: I54b550e6f972a13f61cbcf15ee1fb9fe3c313d7b