Demo app: Update concatenation terminology to use "playlist"

PiperOrigin-RevId: 326401528

demos/main/src/main/java/com/google/android/exoplayer2/demo/PlayerActivity.java

@@ -520,7 +520,7 @@ public class PlayerActivity extends AppCompatActivity
     @Override
     public AdsLoader getAdsLoader(Uri adTagUri) {
       if (mediaItems.size() > 1) {
-        showToast(R.string.unsupported_ads_in_concatenation);
+        showToast(R.string.unsupported_ads_in_playlist);
         releaseAdsLoader();
         return null;
       }

demos/main/src/main/res/values/strings.xml

@@ -51,7 +51,7 @@
 
   <string name="sample_list_load_error">One or more sample lists failed to load</string>
 
-  <string name="unsupported_ads_in_concatenation">Playing sample without ads, as ads are not supported in concatenations</string>
+  <string name="unsupported_ads_in_playlist">Playing without ads, as ads are not supported in playlists</string>
 
   <string name="download_start_error">Failed to start download</string>