Dap: Adds inactive source contents view

This patch adds an informative "Device Unavailable..." contents view for
devices which are present but cannot be connected to for whatever
reason, along with implementing the necessary overrides in the Mtp and
MassStorage Sources.

Related changes:
- DapSync.Enabled checks IsConnected
- RemovableSource.CanUnmap now virtual
- DapSource.CanUnmap checks IsConnected
- DapSource.CanImport checks IsConnected