Exempt VPN apps from BG-FGS-launch restriction.

Exempt apps with OP_ACTIVATE_VPN or OP_ACTIVATE_PLATFORM_VPN.

Bug: 182502553.
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java#testFgsStartVpn

Change-Id: I0590a0b69a3081fdcd2c9aefe9c00e0e84d41740