Move mSessionKey into Ikev2VpnRunner and make it to be a final var

mSessionKey is initialized in the constructor of Ikev2VpnRunner,
move it into Ikev2VpnRunner and make it to be a final var can
make sure that mSessionKey is initialized before someone access
it.

Bug: 191413541
Test: atest FrameworksNetTests:VpnTest
Change-Id: If7eb7e0ec8ab42b17a5636aeced65385e7849b6a
(cherry picked from commit 1cb9f618)
Merged-In: If7eb7e0ec8ab42b17a5636aeced65385e7849b6a