Snapshot merge progress percentage

Compute and print the progress percentage of the dm-snapshot devices
merging after an update is marked as successful.
All the snapshot devices are queried for their update states and all
their allocated, metadata and total sectors are summed as if the system
consists of a single big snapshot device, as well as the initial
allocated sectors to keep trace of the absolute progress. Then the merge
percentage is computed on the numbers for this fake snapshot device.

Bug: 139088917
Test: manual OTA
Change-Id: Ia9a51742105c90b7fd043f3cbf4249877def7f08
Signed-off-by: Alessio Balsini <balsini@google.com>