Clear the IBinder before calling unbindService

The IBinder becomes invalid when unbindService() is called. Clear the
reference to it to prevent the caller from accessing an already closed
Binder.

Bug: 69008332
Test: build and run on a device manually
Change-Id: Id3340ac2799afaa1ea131f116b803de029161172
diff --git a/services/core/java/com/android/server/ServiceWatcher.java b/services/core/java/com/android/server/ServiceWatcher.java
index f20ca43..f4238f2 100644
--- a/services/core/java/com/android/server/ServiceWatcher.java
+++ b/services/core/java/com/android/server/ServiceWatcher.java
@@ -282,6 +282,7 @@
         mBoundUserId = UserHandle.USER_NULL;
         if (component != null) {
             if (D) Log.d(mTag, "unbinding " + component);
+            mBoundService = null;
             mContext.unbindService(this);
         }
     }