Reduce priority of merge threads

Currently, when daemon is spin up, it runs at the highest
priority with nice value set to -20. This can potetially
lead to a problem in a busy system especially after OTA
reboot when all the merge threads are running in parallel.

Now that we reduced the number of merge threads in-flight
to 2, we reduce the priority as well by setting the nice
value to -5. The other threads which serve I/O's
from dm-user (from root filesystem) still runs at higher
priority. We need this because post OTA reboot, these
threads serve I/O's until merge is completed.

Merge threads on the other hand can run at a relatively
lower priority. We need to make sure that there
is always forward progress even in a busy system
and hence set the priority to -5 as compared
to default value of 0.

No boot time regressions observed.

Output of NICE value of merge and worker threads post OTA reboot:

1 S     0   427   451     1 0  39 -20  64 2314640 dev_r+ ?      00:00:00 8
1 S     0   427   486     1 4  39 -20  64 2314640 dev_r+ ?      00:00:02 8
1 S     0   427   487     1 4  39 -20  64 2314640 dev_r+ ?      00:00:02 8
1 S     0   427   488     1 3  39 -20  64 2314640 dev_r+ ?      00:00:02 8
5 R     0   427   634     1 1  24  -5  64 2314640 0    ?        00:00:00 8
5 R     0   427   935     1 5  24  -5  64 2314640 0    ?        00:00:02 8

Bug: 237490659
Test: Full and incremental OTA
Ignore-AOSP-First: cherry-pick from aosp
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I6791dd72ccd8cd5bba6eff663bb3f9598bce7ed2