libsnapshot: reorder COW ops vector

Reorder COW ops vector based on merge sequence. We don't
need additional vector to be stored in memory.

Memory usage for a full OTA on Pixel:

Without Patch:
RssAnon:       61020 kB

With Patch:
RssAnon:	   51112 kB

Bug: 237490659
Test: OTA on Pixel
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I543dd73acfa7cf4e57379e82bc184e943072e7c8