Set taskprofile to snapshot merge thread

Assign CPUSET_SP_BACKGROUND taskprofile to snapshot merge threads.
This will ensure that the threads will not run on big cores.

Additionally, reduce the flushing of data to 1MB after merging REPLACE ops.

No major regression observed on snashot merge time.

On Pixel 6 for incremental OTA of 500M, snapshot merge time increased
from 72 seconds to 76 seconds after this patch.

Bug: 311233916
Test: Full and incremental OTA on Pixel 6 - Verify merge threads not on big cores
Change-Id: I455afdac0b77227869d846d0c4472ea9eb34c41c
Signed-off-by: Akilesh Kailash <akailash@google.com>