settings: add crostini export/import

Backup and restore buttons will be shown in the
crostini details settings subpage when the CrostiniBackup
feature flag is enabled, and when this feature is not restricted
by enterprise policy.

Bug: 912638
Change-Id: I4fb0f04a88df98343a8fa5bef84d197a59660ae2
Reviewed-by: Joel Hockey <>
Reviewed-by: Demetrios Papadopoulos <>
Commit-Queue: Joel Hockey <>
Cr-Commit-Position: refs/heads/master@{#632523}
14 files changed