libsnapshot: Allow forward merge on FDR

If forward merge indicator exists at unverified state during FDR,
we are allowed to initiate merge and finish it before wiping userdata.

Test: downgrades
Test: update with data wipe
Test: vts_libsnapshot_test
Bug: 152094219
Change-Id: I151f1af4aca446cc52e669c5c3ea2b2e55acb468