moved ParcelFileDescriptor to util package.
cleanup.
pop ongoing notification when a device was mounted.
add unmount Broadcast.
allow StorageDevices to unmount.
unmount the device when the user clicks the notification.
fix read-only crash.
remove expects the key.
simplified event handling.

by have the StorageManager notify the DocumentProvider of new roots directly instead of taking the: StorageManager -> DeviceManager -> DocumentProviderImpl -> DocumentProvider route.
Merge branch 'feature/issue-26-notification' into dev

fixes issue #26.