Switch from NettyChannelBuilder to ManagedChannelBuilder

[garrettjonesgoogle]

Specifically, in https://github.com/googleapis/gax-java/blob/master/src/main/java/com/google/api/gax/grpc/InstantiatingChannelProvider.java . NettyChannelBuilder is @Experimental so we shouldn't depend on it directly; all of the same functionality should be available in ManagedChannelBuilder.