snapshot: helper to get access snapshot_state path

/metadata/ota/snapshot_state is a temporary file that keeps track of
snapshot merge operations across reboots.

Bug: 138817833
Test: m
Change-Id: I9785db3f73e1575b8ac8a522d938d09f8948e1b0
Signed-off-by: Alessio Balsini <balsini@google.com>