create_snapshot: Enable v3 writer + variable block size

1: Move to v3 COW writer

2: Enable variable block size. Default compression set to lz4
with compression factor 64KiB

3: Prepare merge sequence so that device can initiate the merge

4: Verify the merge order

Bug: 319309466

Test: On Pixel 6

This was tested on live builds where the actual builds/testing
is done on CI.

Patch-Create+Apply = Create the snapshot patches between two
builds and apply them to the device

Branch(main)            Patch-Creation+Apply  Snapshot-size
=============================================================
Build-1 -> Build-2      14 seconds            160MB

Build-2 -> Build-3      21 seconds            331MB

Build-3 -> Build-4      30 seconds            375MB

Build X -> Build X      3 seconds             8MB

Change-Id: I96437032de029d89de62ba11fe37d9287b0a4071
Signed-off-by: Akilesh Kailash <akailash@google.com>