[android] Explain why the TTS button doesn't work when no engine installed

Signed-off-by: Harry Bond <me@hbond.xyz>

android/app/src/main/java/app/organicmaps/widget/menu/NavMenu.java

@@ -3,6 +3,7 @@ package app.organicmaps.widget.menu;
 import android.location.Location;
 import android.util.Pair;
 import android.view.View;
+import android.widget.Toast;
 import androidx.annotation.NonNull;
 import androidx.appcompat.app.AppCompatActivity;
 import androidx.core.content.ContextCompat;
@@ -10,6 +11,7 @@ import app.organicmaps.MwmApplication;
 import app.organicmaps.R;
 import app.organicmaps.sdk.routing.RoutingInfo;
 import app.organicmaps.sdk.sound.TtsPlayer;
+import app.organicmaps.sdk.util.Config;
 import app.organicmaps.sdk.util.StringUtils;
 import app.organicmaps.util.Graphics;
 import app.organicmaps.util.ThemeUtils;
@@ -125,6 +127,8 @@ public class NavMenu
 
   private void onTtsClicked()
   {
+    if (!TtsPlayer.isReady())
+      Toast.makeText(mActivity, R.string.pref_tts_no_system_tts_short, Toast.LENGTH_SHORT).show();
     TtsPlayer.setEnabled(!TtsPlayer.isEnabled());
     refreshTts();
   }

android/app/src/main/res/values/strings.xml

@@ -941,4 +941,5 @@
     <!-- Title of the bottom sheet when sharing the track -->
     <string name="share_track">Share Track</string>
     <string name="delete_track_dialog_title">Delete %s?</string>
+    <string name="pref_tts_no_system_tts_short">No text-to-speech engine found, check the app settings</string>
 </resources>

android/sdk/src/main/java/app/organicmaps/sdk/sound/TtsPlayer.java

@@ -220,7 +220,7 @@ public enum TtsPlayer
     }
   }
 
-  private static boolean isReady()
+  public static boolean isReady()
   {
     return (INSTANCE.mTts != null && !INSTANCE.mUnavailable && !INSTANCE.mInitializing);
   }