Reset binder during unbinding

On some rare case, we may encounter a DeadObjectException on wrap()
when a new test app is installed. This happens because we reuse the
binder from the previous bindToService() call. Always reset the
binder to make sure this doesn't happen.

Bug: 172780686
Test: run storeRebootEscrowKey twice

Change-Id: I36150ef39f870e38a5f1b4bd814465d70f8dbf85

services/core/java/com/android/server/locksettings/ResumeOnRebootServiceProvider.java

@@ -120,6 +120,7 @@ public class ResumeOnRebootServiceProvider {
             if (mServiceConnection != null) {
                 mContext.unbindService(mServiceConnection);
             }
+            mBinder = null;
         }
 
         /** Bind to the service */