Complete TunerTimeFilter implementation

This CL also set all the successfully closed instance to null
to avoid duplicate closing on the same instance.

Test: make
Bug: 159067322
Bug: 176190508
Change-Id: Idf445641b47f4d8ab4230f30cf6ea3abd4226cbc
diff --git a/services/tuner/TunerFrontend.cpp b/services/tuner/TunerFrontend.cpp
index b85e58b..c62ba6b 100644
--- a/services/tuner/TunerFrontend.cpp
+++ b/services/tuner/TunerFrontend.cpp
@@ -278,11 +278,13 @@
     }
 
     Result status = mFrontend->close();
-    if (status == Result::SUCCESS) {
-        return Status::ok();
-    }
+    mFrontend = NULL;
+    mFrontend_1_1 = NULL;
 
-    return Status::fromServiceSpecificError(static_cast<int32_t>(status));
+    if (status != Result::SUCCESS) {
+        return Status::fromServiceSpecificError(static_cast<int32_t>(status));
+    }
+    return Status::ok();
 }
 
 Status TunerFrontend::getStatus(const vector<int32_t>& statusTypes,