Make the virtual display's window translucent (flutter#9828)

By default alpha is not preserved for the VD's contents.
We make the window translucent as a side effect of doing so is preserving alpha.
There should not be extra performance cost for setting the window to be translucent as there is only a single window within the VD.

Fixes flutter/flutter#33756

shell/platform/android/io/flutter/plugin/platform/SingleViewPresentation.java

@@ -96,7 +96,11 @@ public SingleViewPresentation(
             Object createParams,
             OnFocusChangeListener focusChangeListener
     ) {
-        super(outerContext, display);
+        // By default alpha is not preserved for the VD's contents.
+        // We make the window translucent as a side effect of doing so is preserving alpha.
+        // There should not be extra performance cost for setting the window to be translucent as
+        // there is only a single window within the VD.
+        super(outerContext, display, android.R.style.Theme_Translucent_NoTitleBar_Fullscreen);
         this.viewFactory = viewFactory;
         this.accessibilityEventsDelegate = accessibilityEventsDelegate;
         this.viewId = viewId;
@@ -109,6 +113,7 @@ public SingleViewPresentation(
         );
     }
 
+
     /**
      * Creates a presentation that will attach an already existing view as
      * its root view.
@@ -124,7 +129,11 @@ public SingleViewPresentation(
             OnFocusChangeListener focusChangeListener,
             boolean startFocused
     ) {
-        super(outerContext, display);
+        // By default alpha is not preserved for the VD's contents.
+        // We make the window translucent as a side effect of doing so is preserving alpha.
+        // There should not be extra performance cost for setting the window to be translucent as
+        // there is only a single window within the VD.
+        super(outerContext, display, android.R.style.Theme_Translucent_NoTitleBar_Fullscreen);
         this.accessibilityEventsDelegate = accessibilityEventsDelegate;
         viewFactory = null;
         this.state = state;