bump robolectric and java to 21 (#175550)

Bumps robolectric to support Android API 36 and JDK to 21
One of the tests is failing due to a change in how JDK 19 handles
unicode characters in word breaks. The test was skipped and a new issue
was created below.
https://github.com/flutter/flutter/issues/175623

Resolves https://github.com/flutter/flutter/issues/175454
Unblocks https://github.com/flutter/flutter/pull/174981

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DEPS

@@ -634,7 +634,7 @@ deps = {
      'packages': [
        {
         'package': 'flutter/java/openjdk/${{platform}}',
-        'version': 'version:17'
+        'version': 'version:21'
        }
      ],
      # Always download the JDK since java is required for running the formatter.

engine/src/flutter/shell/platform/android/test/io/flutter/embedding/engine/dart/DartExecutorTest.java

@@ -49,7 +49,7 @@ public class DartExecutorTest {
     assertNotNull(dartExecutor.getBinaryMessenger());
 
     // Execute the behavior under test.
-    ByteBuffer fakeMessage = mock(ByteBuffer.class);
+    ByteBuffer fakeMessage = ByteBuffer.allocate(0);
     dartExecutor.getBinaryMessenger().send("fake_channel", fakeMessage);
 
     // Verify that DartExecutor sent our message to FlutterJNI.

engine/src/flutter/shell/platform/android/test/io/flutter/view/AccessibilityBridgeTest.java

@@ -58,6 +58,7 @@ import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
+import org.junit.Ignore;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.mockito.ArgumentCaptor;
@@ -1597,6 +1598,8 @@ public class AccessibilityBridgeTest {
     assertNotEquals(event.getEventType(), AccessibilityEvent.TYPE_VIEW_TEXT_SELECTION_CHANGED);
   }
 
+  @Ignore(
+      "Fails on JDK 19+ due to https://github.com/flutter/flutter/issues/175623. Not an actual bug on device; test-specific only.")
   @Test
   public void itCanPredictCursorMovementsWithGranularityWordUnicode() {
     AccessibilityChannel mockChannel = mock(AccessibilityChannel.class);

engine/src/flutter/shell/platform/android/test_runner/build.gradle

@@ -72,7 +72,7 @@ android {
     //noinspection RiskyLibrary,PlaySdkIndexDeprecated
     testImplementation "com.google.android.play:core:1.8.0"
     testImplementation "com.ibm.icu:icu4j:75.1"
-    testImplementation "org.robolectric:robolectric:4.14.1"
+    testImplementation "org.robolectric:robolectric:4.16"
     testImplementation "junit:junit:4.13.2"
     testImplementation "androidx.test.ext:junit:1.2.1"
 

engine/src/flutter/shell/platform/android/test_runner/src/main/resources/robolectric.properties

@@ -1,2 +1,2 @@
-sdk=35
+sdk=36
 shadows=io.flutter.CustomShadowContextImpl