Tell Android which SemanticsNodes are focusable (flutter#4067)

A node is considered focusable if it contains information that is interesing to the user. A node that doesn't add any semantic information of its own should not be focusable. It's expected that such a node has children, who have smeantics information and are therefore focusable.

Fixes flutter/flutter#11179

shell/platform/android/io/flutter/view/AccessibilityBridge.java

@@ -95,6 +95,7 @@ public AccessibilityNodeInfo createAccessibilityNodeInfo(int virtualViewId) {
         result.setPackageName(mOwner.getContext().getPackageName());
         result.setClassName("Flutter"); // TODO(goderbauer): Set proper class names
         result.setSource(mOwner, virtualViewId);
+        result.setFocusable(object.isFocusable());
 
         if (object.parent != null) {
             assert object.id > 0;
@@ -144,7 +145,6 @@ public AccessibilityNodeInfo createAccessibilityNodeInfo(int virtualViewId) {
         }
         if ((object.actions & SEMANTICS_ACTION_INCREASE) != 0
                 || (object.actions & SEMANTICS_ACTION_DECREASE) != 0 ) {
-            result.setFocusable(true);
             result.setClassName("android.widget.SeekBar");
             if ((object.actions & SEMANTICS_ACTION_INCREASE) != 0) {
                 result.addAction(AccessibilityNodeInfo.ACTION_SCROLL_FORWARD);
@@ -509,6 +509,10 @@ SemanticsObject hitTest(float[] point) {
             return this;
         }
 
+        boolean isFocusable() {
+            return flags != 0 || label != null || (actions & ~SEMANTICS_ACTION_SCROLLABLE) != 0;
+        }
+
         void updateRecursively(float[] ancestorTransform, Set<SemanticsObject> visitedObjects, boolean forceUpdate) {
             visitedObjects.add(this);