Stop to remove device rows at MtpDocumentsProvider.

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
1 file changed