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
1 file changed