Always remove the LocalServices interface before adding a new one.

Another test appears to be not cleaning up the interfaces registered
with LocalServices, leaving the process in a bad state. To make tests
more robust, remove any existing service before registering a new one,
instead of assuming a clean state.

Bug: 432481295
Test: atest services/tests/servicestests/src/com/android/server/net/NetworkPolicyManagerServiceTest.java
Flag: TEST_ONLY
Change-Id: Icf3ee94e184f30103cac5b7ce617e46851f99a1b