Add the default a11y service to the target shortcut service when turns

on the default shortcut.

Bug: 294575237
Test: atest AccessibilityShortcutControllerTest
Test: manual
- Test steps:
    - In SUW landing page, use the volume keys to trigger an accessibility shortcut
    - Given the default accessibility service is TalkBack
    - Click Turn on when seeing Turn on TalkBack shortcut? warning dialog
    - [Expected] TalkBack component name is added to
      Settings.Secure.ACCESSIBILITY_SHORTCUT_TARGET_SERVICE by running the
      adb command `adb shell settings get secure accessibility_shortcut_target_service`
      and verify the output contains Talkback component name.
    - Go to Vision Settings
    - Click Select to Speak
    - Set up volume keys as the shortcut to trigger Select to Speak
    - Use the volume keys to trigger the accessibility service
    - [Expected] See the dialog letting the user to choose whether to
      turn on Select to Speak or TalkBack.

Change-Id: I1482240e8fbd778ad760a0a0c07a3c05bb2ed817