Clear reference to the IIntentReceiver in order to avoid memory leak

When using sendOrderedBroadcast(..) with a BroadcastReceiver the
BroadcastReceiver instance was not released. The reason for this was that
the resultTo field in the BroadcastRecord kept a reference until it was pushed
out of the mBroadcastHistory. This reference in turn kept a reference to the
process side IIntentReceiver (implemented in ReceiverDispatcher$InnerReceiver).
This in turn had a strong reference (through mStrongRef) to the Context.

In order to keep the debug output the resultTo is also kept as a String in the
new resultToString variable.

Change-Id: I4382a22a541c27b3694fb2b78a04ee820b235f8f