Refactor update status management as protobuf

Convert UpdateState and introduce SnapshotUpdateStatus to protobuf to
simplify its access.
In addition, SnapshotUpdateStatus also stores the sum of all the
snapshot sector information. This additional data is used to improve the
merge progress progress.

Bug: 139088917
Test: manual OTA
Change-Id: Ic777d50244c1afa1cdd75fe9b2ffc6dd9ba19ade
Signed-off-by: Alessio Balsini <balsini@google.com>