Previously MtpDocumentsProvider opens a device just after device is
connected to Android. But MtpDocumentsProvider should open MTP device on
demand so that other applications can open device if user starts to use
the application before using MtpDocumentsProvider.
BUG=26625708
Change-Id: I6083b8c7cef49ee6e9fb0d15ca4adc129734f3eb