Encode scroll motion events in the Android touch processor (#8149)

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

@@ -150,7 +150,9 @@ public class AndroidTouchProcessor {
     // Mouse hover support is not implemented for API < 18.
     boolean isPointerEvent = Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR2
         && event.isFromSource(InputDevice.SOURCE_CLASS_POINTER);
-    if (!isPointerEvent || event.getActionMasked() != MotionEvent.ACTION_HOVER_MOVE) {
+    boolean isMovementEvent = (event.getActionMasked() == MotionEvent.ACTION_HOVER_MOVE
+        || event.getActionMasked() == MotionEvent.ACTION_SCROLL);
+    if (!isPointerEvent || !isMovementEvent) {
       return false;
     }
 
@@ -181,7 +183,9 @@ public class AndroidTouchProcessor {
 
     int pointerKind = getPointerDeviceTypeForToolType(event.getToolType(pointerIndex));
 
-    int signalKind = PointerSignalKind.NONE;
+    int signalKind = event.getActionMasked() == MotionEvent.ACTION_SCROLL
+        ? PointerSignalKind.SCROLL
+        : PointerSignalKind.NONE;
 
     long timeStamp = event.getEventTime() * 1000; // Convert from milliseconds to microseconds.
 
@@ -242,8 +246,13 @@ public class AndroidTouchProcessor {
 
     packet.putLong(pointerData); // platformData
 
-    packet.putDouble(0.0); // scroll_delta_x
-    packet.putDouble(0.0); // scroll_delta_y
+    if (signalKind == PointerSignalKind.SCROLL) {
+      packet.putDouble(-event.getAxisValue(MotionEvent.AXIS_HSCROLL)); // scroll_delta_x
+      packet.putDouble(-event.getAxisValue(MotionEvent.AXIS_VSCROLL)); // scroll_delta_y
+    } else {
+      packet.putDouble(0.0);  // scroll_delta_x
+      packet.putDouble(0.0);  // scroll_delta_x
+    }
   }
 
   @PointerChange
@@ -269,6 +278,9 @@ public class AndroidTouchProcessor {
     if (maskedAction == MotionEvent.ACTION_CANCEL) {
       return PointerChange.CANCEL;
     }
+    if (maskedAction == MotionEvent.ACTION_SCROLL) {
+      return PointerChange.HOVER;
+    }
     return -1;
   }