a11y: Add cursor following mode dialog

This a pure UI change that adds a new magnification cursor following
mode dialog behind a flag. The framework support will be added
separately later.

There are 3 modes as the following:
  - continuous mode
  - center mode
  - edge mode

It also renames magnification mode dialog xml file for general purpose
within accessibility.

NO_IFTTT=linter not working

Bug: b/388335935
Flag: com.android.settings.accessibility.enable_magnification_cursor_following_dialog
Test: SettingsRoboTests:com.android.settings.accessibility.ToggleScreenMagnificationPreferenceFragmentTest &&
      SettingsRoboTests:com.android.settings.accessibility.MagnificationModePreferenceControllerTest &&
      SettingsRoboTests:com.android.settings.accessibility.MagnificationCursorFollowingModePreferenceControllerTest
Change-Id: If2672186faf7443cc210d79630b1ea4f3808d7e4

src/com/android/settings/accessibility/MagnificationModePreferenceController.java

@@ -176,8 +176,12 @@ public class MagnificationModePreferenceController extends BasePreferenceControl
                 mContext, mModeInfos, this::onMagnificationModeSelected);
 
         final View headerView = LayoutInflater.from(mContext).inflate(
-                R.layout.accessibility_magnification_mode_header,
-                getMagnificationModesListView(), /* attachToRoot= */false);
+                R.layout.accessibility_dialog_header, getMagnificationModesListView(),
+                /* attachToRoot= */false);
+        final TextView textView = Preconditions.checkNotNull(headerView.findViewById(
+                R.id.accessibility_dialog_header_text_view));
+        textView.setText(
+                mContext.getString(R.string.accessibility_magnification_area_settings_message));
         getMagnificationModesListView().addHeaderView(headerView, /* data= */null,
                 /* isSelectable= */false);