Merge cherrypicks of ['googleplex-android-review.googlesource.com/24946343'] into sc-v2-platform-release.

Change-Id: If28221b85b2d074655f4433a472de317433848c5
diff --git a/jni/com_android_bluetooth_btservice_AdapterService.cpp b/jni/com_android_bluetooth_btservice_AdapterService.cpp
index 62c7d52..e26171e 100644
--- a/jni/com_android_bluetooth_btservice_AdapterService.cpp
+++ b/jni/com_android_bluetooth_btservice_AdapterService.cpp
@@ -582,6 +582,7 @@
     }
     vm->DetachCurrentThread();
     sHaveCallbackThread = false;
+    callbackEnv = NULL;
   }
 }