random_binder: don't try to get ServiceManager on host

There is no ServiceManager binder on host, so don't try to get it.

Alternative considered:
setDefaultServiceManager(sp<FakeServiceManager>::make());
But if that's called after the process already called
setDefaultServiceManager or defaultServiceManager, it will abort. So
existing users of libbinder_random_parcel would break.

Flag: EXEMPT test only
Test: binder_parcel_fuzzer # host and device
Bug: 407122315
Change-Id: I2a36ac54bac5f088d9a3e8a2f359b7a5acaddc16