Finish current music when Sleep Timer stops (#629)

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.

app/src/main/res/values-zh-rCN/strings.xml

@@ -294,4 +294,5 @@
     <string name="sort_order_artist">艺术家</string>
     <string name="sort_order_album">专辑</string>
     <string name="sort_order_year">年份</string>
+    <string name="finish_current_music_sleep_timer">播完当前音乐</string>
 </resources>