IServiceManager: @nullable IBinder

These binders are expected to be null (for getService/checkService),
however '@nullable' only affects the C++/NDK aidl backends, and these
APIs are only used in libbinder IServiceManager.cpp. This wasn't noticed
since error is collapsed into nullptr for most APIs. However, in
waitForService, this broke things, since it was expecting error only
when there is a real error.

Fixes: 144534032
Test: boot and check waitForService works appropriately when service
    starts too late. Up until now, surprisingly, users of this function
    have all been lucky or haven't noticed an issue.

Change-Id: I138c9916253d56a0dcf7dd02b8bf46e84a3c7a33