SplashScreen: use Androidx library.

Improvement for Android 12+.
Fixes #1365.

src/main/java/de/dennisguse/opentracks/TrackListActivity.java

@@ -38,6 +38,7 @@ import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.appcompat.widget.SearchView;
 import androidx.core.content.ContextCompat;
+import androidx.core.splashscreen.SplashScreen;
 import androidx.cursoradapter.widget.ResourceCursorAdapter;
 import androidx.loader.app.LoaderManager;
 import androidx.loader.content.CursorLoader;
@@ -158,8 +159,7 @@ public class TrackListActivity extends AbstractTrackDeleteActivity implements Co
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
-        // Reset theme after splash
-        setTheme(R.style.ThemeCustom);
+        SplashScreen.installSplashScreen(this);
 
         super.onCreate(savedInstanceState);
         setDefaultKeyMode(DEFAULT_KEYS_SEARCH_LOCAL);

src/main/java/de/dennisguse/opentracks/publicapi/AbstractAPIActivity.java

@@ -6,6 +6,7 @@ import android.widget.Toast;
 
 import androidx.annotation.Nullable;
 import androidx.appcompat.app.AppCompatActivity;
+import androidx.core.splashscreen.SplashScreen;
 
 import de.dennisguse.opentracks.R;
 import de.dennisguse.opentracks.services.TrackRecordingService;
@@ -30,7 +31,10 @@ public abstract class AbstractAPIActivity extends AppCompatActivity {
 
     @Override
     protected void onCreate(@Nullable Bundle savedInstanceState) {
+        SplashScreen splashScreen = SplashScreen.installSplashScreen(this);
         super.onCreate(savedInstanceState);
+        splashScreen.setKeepOnScreenCondition(() -> true );
+
         if (PreferencesUtils.isPublicAPIenabled()) {
             Log.i(TAG, "Received and trying to execute requested action.");
             new TrackRecordingServiceConnection(serviceConnectedCallback)