VpnPreferenceControllerTest converted to JUnit

global settings string set to guarantee pre-test environment.
PreferenceManager and PreferenceScreen instantiated instead of mocks,
and mock preference inserted with getKey function overridden.

Bug: 176821216
Test: atest -c VpnPreferenceControllerTest
Change-Id: I94a881df13d1b35126de10f561d657136e4fed7f
Merged-In: I94a881df13d1b35126de10f561d657136e4fed7f
(cherry picked from commit 7361f855)