Camera: Fix unstable test result on testpreviewcallback

The callback is called at unexpected timing.
To avoid this issue, we need to wait for a while before going
to the next test.

Change-Id: I1d17cbd4c6678eca399829828119c85ba805b243
diff --git a/tests/tests/hardware/src/android/hardware/cts/CameraTest.java b/tests/tests/hardware/src/android/hardware/cts/CameraTest.java
index a261914..4200e2b 100644
--- a/tests/tests/hardware/src/android/hardware/cts/CameraTest.java
+++ b/tests/tests/hardware/src/android/hardware/cts/CameraTest.java
@@ -409,6 +409,13 @@
             waitForPreviewDone();
             assertTrue(mPreviewCallbackResult);
             mCamera.stopPreview();
+            try {
+                // Wait for a while to throw away the remaining preview frames.
+                Thread.sleep(1000);
+            } catch(Exception e) {
+                // ignore
+            }
+            mPreviewDone.close();
         }
         terminateMessageLooper();
     }