libsnapshot: Tune readahead during OTA for source and COW block device

Scanning of partitions post OTA leads to memory pressure. Tune
the read-ahead of source and COW block device. This is currently
set to 32KB.

This reduces Inactive(file) and Active(file) usage during entire
duration of boot post OTA.

On Pixel 6: For incremental OTA ~400M. During boot:

                            Without-patch         With-patch    Delta
			    --------------------------------------------
1: Peak Inactive(file):     4469MB                3118MB        ~30%

2: Peak Active(file):       985MB                 712MB         ~27%

No regression observed on boot time.

Additionally, cut down the number of threads to verify the partitions.

Bug: 311233916
Test: Incremental OTA on Pixel 6
Change-Id: I0b842776c36fa089c39c170fa7bf0f246e16636d
Signed-off-by: Akilesh Kailash <akailash@google.com>