Fix crash when deleting multiple files.

When deleting files, MtpDocumentsProvider clears LoadingTask in
DocumentsLoader to update directory contents list. Previously it can
clear ongoing task, and it skips calling Mapper#stopAddingDocuments.
Since Mapper#startAddingDocuments and Mapper#stopAddingDocuments must be
called 1 to 1, it causes precondition check failure at the next call of
Mapper#startAddingDocuments.

Change-Id: I23e2b117da826297e45404be4db4cc29f96e5510
Fix: 28076320