[sdk-platform-java] Investigate using a Mock gRPC Channel for InstantiatingGrpcChannelProviderTest

Following the flaky test reported in here: https://github.com/googleapis/sdk-platform-java/issues/2852, we discovered that some unit tests are creating a gRPC channel. Ideally, we would like to use a mock or stub instead of a real gRPC channel.

This reduces the change for flaky behavior and also would help speed up the tests (creating a channel is ~500ms).

This ticket is to investigate whether we can use a mock or stub instead of an actual channel and implement the change.