Add missing trackpad enum to android and web (flutter/engine#31429)

engine/src/flutter/lib/web_ui/lib/pointer.dart

@@ -22,6 +22,7 @@ enum PointerDeviceKind {
   mouse,
   stylus,
   invertedStylus,
+  trackpad,
   unknown
 }
 

engine/src/flutter/shell/platform/android/io/flutter/embedding/android/AndroidTouchProcessor.java

@@ -45,6 +45,7 @@ public class AndroidTouchProcessor {
     PointerDeviceKind.MOUSE,
     PointerDeviceKind.STYLUS,
     PointerDeviceKind.INVERTED_STYLUS,
+    PointerDeviceKind.TRACKPAD,
     PointerDeviceKind.UNKNOWN
   })
   private @interface PointerDeviceKind {
@@ -52,7 +53,8 @@ public class AndroidTouchProcessor {
     int MOUSE = 1;
     int STYLUS = 2;
     int INVERTED_STYLUS = 3;
-    int UNKNOWN = 4;
+    int TRACKPAD = 4;
+    int UNKNOWN = 5;
   }
 
   // Must match the PointerSignalKind enum in pointer.dart.