Allow browsing sdcard when there is an unreadable file in it (eg .android_secure)

Summary:
On my Samsung Galaxy S5 mini (Android 6.0.1) browsing the sdcard is not possible because there
is a hidden file .android_secure that is not readable by a normal user.
For such a file File.exists() returns false while File.listFiles() does return it
and SftpSubsystem.writeAttrs() throws an exception

Test Plan:
On a device where the SDCard cannot be browsed (I think its API 23 and up)
install kdeconnect with this patch and verify that the SDCard can now be browsed successfully

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, nicolasfella, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17644