libsnapshot: Use a two-phase merge for VABC.

If a partition shrinks in an update, it must be merged before any other
partitions. Otherwise, a copy operation may source from the tail of the
shrunk partition, which could be overwritten by a merge operation in
another partition.

This patch adds a "MergePhase" indicator to the update status that is
valid only when the state is MERGING. Partitions that shrink are merged
first, and the phase will be FIRST_PHASE. Once ProcessUpdateState() has
determined that all first-phase snapshots are merged, it will switch to
SECOND_PHASE and remaining snapshots can start merging.

Otherwise, there is no change to the merge algorithm. The phase split is
an implementation detail and not exposed to update_engine.

Bug: 177935716
Test: vts_libsnapshot_test
Change-Id: I06043f8e3b81bdecefb6a4b5944a97b7086eeb49