base

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