Add '/system/bin/vdc volume reset' command

It will be used during the userspace reboot to unmount all emulated
storages, which is a prerequisite to unmount+remount ext4 userdata into
checkpoint mode.

Test: adb shell /system/bin/vdc checkpoint startCheckpoint 1
Test: adb reboot userspace
Bug: 143970043
Bug: 135984674
Change-Id: I07491bc4be2f30debe21b764766c9def68548e16