Fix Sheet native blocking issue and update samples.

1. Conditionally add a background blocker in Sheet.java if Form.activePeerCount > 0.
2. Update KotlinUiTest.kt to demonstrate Sheet overlay.
3. Update BrowserComponentScreenshotTest.java to demonstrate Sheet over BrowserComponent.
4. Verify logic with unit tests (SheetPeerBlockingTest).

Author: google-labs-jules[bot]

scripts/hellocodenameone/common/src/main/java/com/codenameone/examples/hellocodenameone/tests/BrowserComponentScreenshotTest.java

@@ -2,6 +2,8 @@
 
 import com.codename1.ui.BrowserComponent;
 import com.codename1.ui.Form;
+import com.codename1.ui.Label;
+import com.codename1.ui.Sheet;
 import com.codename1.ui.layouts.BorderLayout;
 import com.codename1.ui.util.UITimer;
 import com.codename1.util.SuccessCallback;
@@ -56,6 +58,10 @@ public void onSucess(BrowserComponent.JSRef result) {
             return;
         }
 
+        Sheet sheet = new Sheet(null, "Overlay Sheet");
+        sheet.getContentPane().add(new Label("This is a sheet covering part of the browser"));
+        sheet.show();
+
         UITimer.timer(2000, false, form, readyRunnable);
         readyRunnable = null;
     }

scripts/hellocodenameone/common/src/main/kotlin/com/codenameone/examples/hellocodenameone/tests/KotlinUiTest.kt

@@ -7,6 +7,7 @@ import com.codename1.ui.Button
 import com.codename1.ui.CheckBox
 import com.codename1.ui.Container
 import com.codename1.ui.Label
+import com.codename1.ui.Sheet
 import com.codename1.ui.Slider
 import com.codename1.ui.TextArea
 import com.codename1.ui.TextField
@@ -51,6 +52,11 @@ class KotlinUiTest : BaseTest() {
 
         kotlinForm.add(accordion)
         kotlinForm.show()
+
+        val sheet = Sheet(null, "Overlay Sheet")
+        sheet.contentPane.add(Label("This is a sheet covering part of the screen"))
+        sheet.show()
+
         return true
     }
 }