Use Android text selection shifting API to handle keyboard backspace (#8956)

Author: GaryQian

shell/platform/android/io/flutter/plugin/editing/InputConnectionAdaptor.java

@@ -5,8 +5,11 @@
 package io.flutter.plugin.editing;
 
 import android.content.Context;
+import android.text.DynamicLayout;
 import android.text.Editable;
+import android.text.Layout;
 import android.text.Selection;
+import android.text.TextPaint;
 import android.view.KeyEvent;
 import android.view.View;
 import android.view.inputmethod.BaseInputConnection;
@@ -24,10 +27,12 @@ class InputConnectionAdaptor extends BaseInputConnection {
     private final Editable mEditable;
     private int mBatchCount;
     private InputMethodManager mImm;
+    private final Layout mLayout;
 
     private static final MethodChannel.Result logger =
         new ErrorLogResult("FlutterTextInput");
 
+    @SuppressWarnings("deprecation")
     public InputConnectionAdaptor(
         View view,
         int client,
@@ -40,6 +45,9 @@ public InputConnectionAdaptor(
         this.textInputChannel = textInputChannel;
         mEditable = editable;
         mBatchCount = 0;
+        // We create a dummy Layout with max width so that the selection
+        // shifting acts as if all text were in one line.
+        mLayout = new DynamicLayout(mEditable, new TextPaint(), Integer.MAX_VALUE, Layout.Alignment.ALIGN_NORMAL, 1.0f, 0.0f, false);
         mImm = (InputMethodManager) view.getContext().getSystemService(Context.INPUT_METHOD_SERVICE);
     }
 
@@ -144,7 +152,8 @@ public boolean sendKeyEvent(KeyEvent event) {
                     return true;
                 } else if (selStart > 0) {
                     // Delete to the left of the cursor.
-                    int newSel = Math.max(selStart - 1, 0);
+                    Selection.extendLeft(mEditable, mLayout);
+                    int newSel = Selection.getSelectionEnd(mEditable);
                     Selection.setSelection(mEditable, newSel);
                     mEditable.delete(newSel, selStart);
                     updateEditingState();