libsnapshot: Place forward merge indicator for wipe

If the device applies a package with FDR, place forward merge indicator
so that on FDR, the snapshots aren't abandoned, and we forward merge
the update.

Test: apply downgrade
Test: apply update with data wipe
Test: vts_libsnapshot_test
Bug: 152094219
Change-Id: Id32aeca380db797cd777321837f146333ae6b33c