Allow direct reads on source device

Allow O_DIRECT reads on source block device.
This will further cut down the Active and Inactive file pages
during partition verification.

On Pixel 6 after incremental OTA - Post OTA reboot:

		Without patch      With patch     Delta
--------------------------------------------------------
Inactive(File):  4992MB             3887MB         ~22%
Active(File):    1465MB             1014MB         ~30%

Boot time however increases from 25 to 30 seconds.

This is not yet enabled. This will be behind a sysprop flag
or for low memory devices and will be enabled later.

Additionally, set the priority of worker threads to normal.
Merge threads priority is reduced. This will help low memory
devices as tested on Pixel watch.

Bug: 311233916
Test: OTA on Pixel 6
Change-Id: Icacdef08d68e28d3062611477703e7cf393a9f10
Signed-off-by: Akilesh Kailash <akailash@google.com>