libsnapshot: Propagate the compression bit across state changes.

This bit was getting lost because InitiateMerge() did not save the
compression bit when overwriting the update state.

Bug: N/A
Test: vts_libsnapshot_test
Test: reboot during merge phase of VABC OTA
Change-Id: I1a2219b501088de352a9c31d4b8b1a3f72d0e159