Use default accessibility notification sound be accessibility volume key shortcut ringtone.

Root cause: The accessibility volume key shortcut plays notification ringtone would misleading the normal users to check notification list when hear this sound.
Solution: Use the default accessibility notification sound instead to avoid users confusing the new notification received. Point to the default notification sound if the sound does not exist.

Bug: 175879613
Test: Manual testing
Change-Id: I5885dcaa5254b6334a95f756bf53b51186c555f5