[kernel][sched] Tweak fair priority curve and preemption / latency.

- Switch to an exponential priority-to-weight curve to yield a ~65%
  bandwidth increase to priority 24 (high priority) over priority 16
  (default priority), and a constant ~10% difference between adjacent
  priorities.
- Tweak the bandwidth adjustment to use the start of the current time
  slice when changing the task rate so that preemptions happen faster
  when high priority work is available.
- Adjust the target latency to provide a time slice more similar to the
  previous scheduler when a high priority thread competes with lower
  priority threads.
- Minor refactor to remove weight-to-priority conversions, which would
  require a fixed-point logarithm to compute the inverse of the
  exponential term.
- Optimize the priority-to-weight conversion using a constant table.

Bug: MTWN-269
Bug: ZX-3504
Test: Detailed tracing of synthetic workload; observe ratios of
      bandwidth between priority 16 and priority 24 tasks.
Change-Id: I596c45cf3d8c94f93c986508f9ec1007624ac9f3