[Android] Reset IME state in TextInputPlugin.clearTextInputClient (#49829)

## Description

This PR calls Android API `InputMethodManager.restartInput` to reset IMEs internal states. Otherwise some IMEs (Gboard for instance) keep reacting based on the previous input configuration until a new configuration is set.

- On Android native, `restartInput` is called in several places, for instance in https://github.com/AndroidSDKSources/android-sdk-sources-for-api-level-34/blob/f2197987748faef78e869662ae1fd039daa22a63/android/widget/TextView.java#L2458.
- On Compose, flutter/flutter#70546 (comment) pointed out where it is called.
- On Flutter, it is called at some point but mainly when another `TextField` is focused (it is mainly called in `setTextInputEditingState`).

## Related Issue

Fixes flutter/flutter#70546.

## Tests

Adds 1 test.

Author: bleroux

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

@@ -569,6 +569,10 @@ void clearTextInputClient() {
     inputTarget = new InputTarget(InputTarget.Type.NO_TARGET, 0);
     unlockPlatformViewInputConnection();
     lastClientRect = null;
+
+    // Call restartInput to reset IME internal states. Otherwise some IMEs (Gboard for instance)
+    // keep reacting based on the previous input configuration until a new configuration is set.
+    mImm.restartInput(mView);
   }
 
   private static class InputTarget {

shell/platform/android/test/io/flutter/plugin/editing/TextInputPluginTest.java

@@ -1128,6 +1128,41 @@ public void setTextInputEditingState_nullInputMethodSubtype() {
     assertEquals(1, testImm.getRestartCount(testView));
   }
 
+  @Test
+  public void clearTextInputClient_alwaysRestartsImm() {
+    // Initialize a general TextInputPlugin.
+    InputMethodSubtype inputMethodSubtype = mock(InputMethodSubtype.class);
+    TestImm testImm = Shadow.extract(ctx.getSystemService(Context.INPUT_METHOD_SERVICE));
+    testImm.setCurrentInputMethodSubtype(inputMethodSubtype);
+    View testView = new View(ctx);
+    TextInputChannel textInputChannel = new TextInputChannel(mock(DartExecutor.class));
+    TextInputPlugin textInputPlugin =
+        new TextInputPlugin(testView, textInputChannel, mock(PlatformViewsController.class));
+    textInputPlugin.setTextInputClient(
+        0,
+        new TextInputChannel.Configuration(
+            false,
+            false,
+            true,
+            true,
+            false,
+            TextInputChannel.TextCapitalization.NONE,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null));
+    // There's a pending restart since we initialized the text input client. Flush that now.
+    textInputPlugin.setTextInputEditingState(
+        testView, new TextInputChannel.TextEditState("", 0, 0, -1, -1));
+    assertEquals(1, testImm.getRestartCount(testView));
+
+    // A restart is always forced when calling clearTextInputClient().
+    textInputPlugin.clearTextInputClient();
+    assertEquals(2, testImm.getRestartCount(testView));
+  }
+
   @Test
   public void destroy_clearTextInputMethodHandler() {
     View testView = new View(ctx);