Add a new API startProvisionedVpnProfileSession()

Add a new API to return a unique key corresponding to a specific
VPN session, so that when VPN sends ACTION_VPN_MANAGER_ERROR
intent, the app can know if this intent is sent for the session
that the app is using.

Bug: 191413541
Test: atest CtsNetTestCases
Change-Id: I372308f7f4d20acf3ea17a322bc5f39e0bac460f