Import PluginVm disk image after downloading: PluginVmImageManager

This adds the last part of the PluginVm setup flow. After the VM
image has been downloaded we need to import it using vm_concierge.

Since vm_concierge does the unzipping itself, this also refactors the
setup flow and replaces unzipping by listening to the corresponding
concierge signal.

Follow-up CLs will add the functionality to cancel the import as well
as additional tests to cover different error cases.

Bug: 947014
Test: browser_tests --gtest_filter="PluginVm*" && unit_tests --gtest_filter="PluginVm*"
Change-Id: If1d038ff9b0e5c9f7ca73ac7c68397620099a30d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611550
Reviewed-by: Olya Kalitova <okalitova@chromium.org>
Commit-Queue: Alex Oldemeier <aoldemeier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660826}
12 files changed