exp/sensor: fix segfault while releasing thread looper

Only the loopers allocated with ALooper_acquire are releasable. We are
depending on the looper associated to the thread which will be
automatically released when the thread is being terminated.

Trying to re-release a looper makes the app crash with segfault.

Change-Id: Ib80549de01f5f44143752c13a82b6b4a2ab5610e
Reviewed-on: https://go-review.googlesource.com/15491
Reviewed-by: David Crawshaw <crawshaw@golang.org>

exp/sensor/android.c

@@ -69,5 +69,6 @@ int GoAndroid_readQueue(int n, int32_t* types, int64_t* timestamps, float* vecto
 void GoAndroid_destroyManager() {
   ASensorManager* manager = ASensorManager_getInstance();
   ASensorManager_destroyEventQueue(manager, queue);
-  ALooper_release(looper);
+  queue = NULL;
+  looper = NULL;
 }