Implement #329.
This commit adds a CheckBox to dialog_sleep_timer, and a curresponding
option `TIMER_FINISH_MUSIC` in PreferenceUtil.
In MusicService, a new flag `pendingQuit` is to indicate if it should
stop after current music stops. A new action `ACTION_PENDING_QUIT` will
set that flag.
Canceling is a little tricky. I must bind MusicService in Dialog
activity to check if `pendingQuit` is set, and show the corresponding
canceling button.
