Cleans up the metadata in MtpDatabase at the first launch after booting.

When rebooting a device, applicaitons lose temporary URI permissions so
we don't need to keep document ID that are not granted persistent URI
permissions.

 1. Check Settings.Global.BOOT_COUNT to find out if it's first time to
    launch MtpDocumentsProvider since booting.
 2. If so, invokes clean up method of MtpDatabase.

BUG=26212981
Change-Id: Ic9a8ca7e7a9cac1ed91fdfb01e9dce14ce819243