Add TEST_MAPPING for VPN related files

Updates on Vpn.java, VpnIkev2Utils.java and VpnProfileStore.java
should be verified with VPN UT and CTS.

Bug: 286302640
Test: atest --test-mapping [src_dir]/Vpn.java and check if
      corresponding tests triggered
Change-Id: Ie08a5305c02c46d43782d4a15bd3ab30e9e2e1a8