libsnapshot: Store index of COW ops vector

Using vector + unordered_map to retrieve the index in
a COW op vector consumes significant memory; this
is a problem especially when there are hundreds
of thousands of operations.

Instead, just store the index of the COW op vector
during pre-processing.

On Pixel, peak memory usage when all the partitions
are mapped:

Without patch:
	RssAnon:	  118804 kB
With path:
	RssAnon:	   55772 kB

Additionally, post OTA reboot, memory usage further goes
down as the partition merge completes.

Bug: 237490659
Test: OTA on Pixel
Ignore-AOSP-First: cherry-pick from aosp
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Icc68a9688ceb89572821cee2dac689779f5e7c11