Retry TTS speaking for a11y shortcut warning dialog

TTS engine might fetch data over the network or prepare data when
binding tts service at first time after boot that may take a few
seconds. Because the dialog is shown only once and on-demand for
enableing accessibility shortcut, it's unable to speak successfully
unless others have bound tts ahead of time.

Bug: 139887992
Test: 1. adb shell settings delete secure accessibility_shortcut_dialog_shown
      2. manual reboot, long press both vol keys and tts speaks
Test: Add 2nd user and long press both vol keys and tts speaks
Test: atest AccessibilityShortcutControllerTest

Change-Id: I6290f0a64d1a51aa46f13e7d88a15a096ddb0fdc