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