Previously MtpDocumentsProvider used to remove device info in the
database when the device is closed. It loses a chance that RootScanner
handles the removal of roots. As a result, notifications for root cursor
does not issued.
The CL stops removing device info at MtpDocumentsProvider, and let
RootScanner remove it.
BUG=26321346
Change-Id: I6aec0cf843a1e5c93e7f67a718cabd26841d9799