pm: use 'available cores' instead of 'available cores + 1' for multi-threaded dexopt

Linux kernel's CFS scheduler does a better job when
the number of workloads is equal to the number of CPU cores.

Since dexopt almost always takes 100% of CPU cycle,
we can expect faster dexopt operation with CFS scheduler
at its best.

Signed-off-by: arter97 <qkrwngud825@gmail.com>