Fix power issues around wifi scans.

- Sending a broadcast indicating when scan requests could be serviced so that
apps don't request scans we won't do anything with.
- Fix our batt stats accounting so we only count it if we send the request to
the driver.

bug: 8868201
bug: 9496690
Change-Id: I64a4f1c294c848ac64c50d8854ed4a6a1a47f603