Release the instance of StkAppService when it is destroyed

The static field 'sInstance' is never updated even after
StkAppService is destroyed. That causes that StkAppService.getInstance()
can return already destroyed service.

Change-Id: Ic2aba47c9dc1e5ff20250869e0188e1eec8334f2
diff --git a/src/com/android/stk/StkAppService.java b/src/com/android/stk/StkAppService.java
index 0b19f9e..8ed6f25 100644
--- a/src/com/android/stk/StkAppService.java
+++ b/src/com/android/stk/StkAppService.java
@@ -362,6 +362,7 @@
             mStkCmdReceiver = null;
         }
         mPowerManager = null;
+        sInstance = null;
         waitForLooper();
         mServiceLooper.quit();
     }