Workaround of Slice not pinned bug when using SliceCallback

When a client registers a SliceCallback, it starts to observe Slice
changes. The observer is running in the main looper and binds Slice
in AsyncTask.

However, when the client unregisters the callback, it unpins the Slice
in the client's thread. Thus, if the AsyncTask has pinned the slice but
hasn't bound it yet, it may be unpinned by the client, and an exception
occurs.

This fix is to unpin the Slice in the same SerialExecutor of AsyncTask,
which is to ensure bindSlice() and unpinSlice() are called in the same
thread one after another without the race condition.

Bug: 157387583
Test: robotest
Change-Id: If5f4b4bc0d8c5a2800cad8ff2afd7084426a6c96