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Rather than having CrostiniManager explicitly attach USB devices when
termina starts, CrosUsbDetector can be a Concierge::VmObserver and do
this for all VMs whether they are started by chrome, or by vmc.

It also observes VmPluginDispatcherClient::OnVmStateChanged RUNNING to
reconnect devices for Plugin VM.

Moved USB functions from CrostiniManager to CrosUsbDetector since
it is the only client and USB is not specific to crostini, but applies
to all VMs.

Bug: b/150341671
Change-Id: Ic14765d3b4d2911ae297b59ab5fbba7212c9957d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386441
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803793}