libsnapshot: Check for valid snapshots based on current slot

We may have snapshot files in /metadata/ota/snapshot/ which ends with
.tmp such as system_a.tmp - This happens if the device
reboots just before `rename` in `WriteStringToFileAtomic`. This
can lead to spurious merge failures.

Log the error and skip these snapshot files. It is ok to skip
as we will still have original snapshot status files since
we are already in the merge path. Additionally, try to remove
these files when snapshot is deleted.

Bug: 292198189
Test: OTA
Change-Id: I5db3dbd5a919b263ae577185de3e7f79a5e9b89a
Signed-off-by: Akilesh Kailash <akailash@google.com>