libsnapshot: Add allow-forward-merge indicator.

The indicator signals that on factory data reset, initiating
merge is allowed.

Test: vts_libsnapshot_test
Test: apply downgrade update
Test: apply update with FDR
Bug: 152094219
Change-Id: Ieecddd198136571dd652ca62c27fe65a12b7b15f
(cherry picked from commit f9e1e163)
Merged-In: Ieecddd198136571dd652ca62c27fe65a12b7b15f