Reduce underruns in screen off, esp. with EQ

Add MonoPipe APIs to specify setpoint.
Use screen state to configure pipe setpoint.
Fix a long-standing bug where pipe sleep time was excessive,
  which interacted poorly with governor and low clock frequencies.
  Now it deducts the elapsed time since last write(),
  which was significant when there was EQ and low clock frequency.

Bug: 6618373
Change-Id: I6f3b0072c2244aeb033ef0795ad164491a164ff5