Avoiding cyclic references when unbinding from a service

Cyclic references can occur between a Service object held by an
application and a ServiceRecord object held by the system server.
A part of the problem is that binders are leaked and since many binders
are implemented by inner classes of services these services are also leaked.
This causes low memory problems. The solution is: When a Service is beeing
destroyed, go through the ServiceRecord's all IntentBindRecord and set its
binder references to null. This allows the binder and the service object to
be garbage collected.

Change-Id: I5a257521964851f34c08ffb3908feaad96b1bafe