sched: honor sync only if CPU is about to goto idle

sync is causing excessive latencies during binder replies as its causing
migration of important tasks to busy CPU. Incase the CPU has a lot of
tasks running, prevent sync from happening

bug: 62806392

Change-Id: I4731bd9fdbf278c4ba55f3abc8c9ed4fa54a31cf
Signed-off-by: Joel Fernandes <joelaf@google.com>