Reduce latency from doze to screen on.

Don't wait for the brightness ramp to complete before reporting
display ready.  Keep track of whether we have any unfinished
brightness changes and take care to grab a wakelock to ensure
they are eventually applied.

Ideally we would rewrite the whole state machine to more carefully
coordinate screen state and brightness changes but that's too
risky for now.

The pixel fairies are having a bad day.

Bug: 17718416

(cherry picked from commit 875f80c2732a3fbe652a6e8fc14031041f791308)

Change-Id: I7a2d8ba4591a12b773653d3dbf86c7db016f967e