libsnapshot: get options from protobuf fields

Update snapshot.cpp to grab estimate_op_buffer_size &
estimate_sequence_buffer_size from update_engine. Update v3 writer to
use these options to size the buffers appropriately.

we probably don't need the fields for merge metrics yet but will leave
it here for now

Test: th
Bug: 313962438
Change-Id: I08252ff66174de9bafaf8dbe9115d9d049084c4c