bugfix: Only register vibrator if it is useable.

All devices prior to Honeycomb were assumed to have a vibrator. Since
Honeycomb, many devices don't have a vibrator.  To maintain backwards
compatibility, Honeycomb+ devices always return a non-NULL Vibrator
object, even if they don't actually have vibration hardware. Instead,
you must specifically query whether the vibrator actually exists.

I tested this on a Gingerbread device to verify that the ternary state-
ment is safe on non-Honeycomb devices.  It was safe for my device.