vibrator HAL uses "default" service name

The getService() and registerAsService() methods of interface objects
now have default parameters of "default" for the service name. HALs
will not have to use any service name unless they want to register
more than one service.

Test: service registers correctly as default

Bug: 33844934
Change-Id: Ic0e9ff98b16bfb66e67222054653b7347442bacb

vibrator/1.0/default/service.cpp

@@ -22,5 +22,5 @@ using android::hardware::vibrator::V1_0::IVibrator;
 using android::hardware::defaultPassthroughServiceImplementation;
 
 int main() {
-    return defaultPassthroughServiceImplementation<IVibrator>("vibrator");
+    return defaultPassthroughServiceImplementation<IVibrator>();
 }