[android] improve auto theming

ported from the big PR. Addresses the issue of the theme not changing when there's no location fix by setting to night at 6pm (still checks when sunset is if you have a location fix)
Signed-off-by: Harry Bond <me@hbond.xyz>

android/app/src/main/java/app/organicmaps/util/ThemeSwitcher.java

@@ -8,6 +8,9 @@ import android.os.Build;
 
 import androidx.annotation.NonNull;
 import androidx.appcompat.app.AppCompatDelegate;
+
+import java.util.Calendar;
+
 import app.organicmaps.Framework;
 import app.organicmaps.MwmApplication;
 import app.organicmaps.R;
@@ -29,30 +32,15 @@ public enum ThemeSwitcher
     @Override
     public void run()
     {
-      String nightTheme = MwmApplication.from(mContext).getString(R.string.theme_night);
-      String defaultTheme = MwmApplication.from(mContext).getString(R.string.theme_default);
-      String theme = defaultTheme;
-      Location last = LocationHelper.from(mContext).getSavedLocation();
-
       boolean navAuto = RoutingController.get().isNavigating() && ThemeUtils.isNavAutoTheme(mContext);
-
-      if (navAuto || ThemeUtils.isAutoTheme(mContext))
-      {
-        if (last == null)
-          theme = Config.getCurrentUiTheme(mContext);
-        else
-        {
-          long currentTime = System.currentTimeMillis() / 1000;
-          boolean day = Framework.nativeIsDayTime(currentTime, last.getLatitude(), last.getLongitude());
-          theme = (day ? defaultTheme : nightTheme);
-        }
-      }
-
-      setThemeAndMapStyle(theme);
+      // Cancel old checker
       UiThread.cancelDelayedTasks(mAutoThemeChecker);
 
       if (navAuto || ThemeUtils.isAutoTheme(mContext))
+      {
         UiThread.runLater(mAutoThemeChecker, CHECK_INTERVAL_MS);
+        setThemeAndMapStyle(calcAutoTheme());
+      }
     }
   };
 
@@ -159,4 +147,30 @@ public enum ThemeSwitcher
     else
       Framework.nativeMarkMapStyle(style);
   }
+
+  /**
+   * Determine light/dark theme based on time and location,
+   * or fall back to time-based (06:00-18:00) when there's no location fix
+   * @return theme_light/dark string
+   */
+  private String calcAutoTheme()
+  {
+    String defaultTheme = mContext.getResources().getString(R.string.theme_default);
+    String nightTheme = mContext.getResources().getString(R.string.theme_night);
+    Location last = LocationHelper.from(mContext).getSavedLocation();
+    boolean day;
+
+    if (last != null)
+    {
+      long currentTime = System.currentTimeMillis() / 1000;
+      day = Framework.nativeIsDayTime(currentTime, last.getLatitude(), last.getLongitude());
+    }
+    else
+    {
+      int currentHour = Calendar.getInstance().get(Calendar.HOUR_OF_DAY);
+      day = (currentHour < 18 && currentHour > 6);
+    }
+
+    return (day ? defaultTheme : nightTheme);
+  }
 }