Call scan callback for ATSC

It's a temp solution for CTS.

Bug: 150952758
Test: atest android.media.tv.tuner.cts.TunerTest
Change-Id: I31242910af39108dca5920bd0892fa16aa6d29be
diff --git a/tv/tuner/1.0/default/Frontend.cpp b/tv/tuner/1.0/default/Frontend.cpp
index 6565746..ae5f587 100644
--- a/tv/tuner/1.0/default/Frontend.cpp
+++ b/tv/tuner/1.0/default/Frontend.cpp
@@ -82,6 +82,10 @@
     ALOGV("%s", __FUNCTION__);
 
     if (mType == FrontendType::ATSC) {
+        FrontendScanMessage msg;
+        msg.isLocked(true);
+        mCallback->onScanMessage(FrontendScanMessageType::LOCKED, msg);
+        mIsLocked = true;
         return Result::SUCCESS;
     }
     if (mType != FrontendType::DVBT) {