Emit onFirstContentfulPaint paint for cached pages

The FCP event is not emitted by GeckoView if the page is loaded from the cache
(either HTTP or back/forward). As a result many things are not properly set up
in Wolvic, like the current active session, resulting in many cases in web
contents not visible on app start.

In those cases we should generate and emit it to get things working.

app/src/common/shared/com/igalia/wolvic/browser/engine/Session.java

@@ -12,6 +12,9 @@
 import android.content.SharedPreferences;
 import android.graphics.Bitmap;
 import androidx.preference.PreferenceManager;
+
+import android.os.Handler;
+import android.os.Looper;
 import android.util.Log;
 import android.view.Surface;
 import android.view.inputmethod.CursorAnchorInfo;
@@ -1364,6 +1367,13 @@ public void onFirstComposite(@NonNull WSession aSession) {
                 for (WSession.ContentDelegate listener : mContentListeners) {
                     listener.onFirstContentfulPaint(aSession);
                 }
+            } else if (!mState.mIsLoading) {
+                new Handler(Looper.getMainLooper()).postDelayed(() -> {
+                    // onFirstContentfulPaint is not emitted sometimes when loading a page from
+                    // the cache. This is a workaround to ensure that the event is emitted.
+                    if (!mFirstContentfulPaint)
+                        onFirstContentfulPaint(aSession);
+                }, 500);
             }
         }
     }