Initialize RadioProxy only once

The previous change only ensured that setResponseFunctions is called
once for HIDL-only devices. Also ensure that other setup like
registering for binder died is also only called once.

Test: atest RILTest
Bug: 225112815
Change-Id: Ifa40e0888b7b5d2ddd0a3e004e490df3fc3bbd1a