Flush after every 2MB merge of replace ops.

This will be in sync with incremental OTA's where the sync
is done every 2MB. This improves performance on devices
with low memory.

Merge times for full OTA may increase by couple of seconds but
that is ok given it decreases the memory footprint.

Bug: 238052097
Test: OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ic2c8d2ffdbdb677e0c4d44e5de68ce8ccf86df34