checkpoint: Fix metadata file contents bug In aosp/787759, Checkpoint changed the way kMetadataCPFile stored the retry count, but didn't account for "-1" as a special case. As a result, Checkpoint no longer reported `needsRollback` to be true when `startCheckpoint(-1)` was called. Bug: none Test: presubmits (lmk if there's something better) Change-Id: I4acb929eb3471409b986f74493993a9f1d84e4a5