Fix order of operations while pulsing the ambient display.

This change fixes several issues in how the display's power state
was coordinated to ensure that the display is not turned on until
the pulse is ready to be shown.  It also greatly simplifies the
wakelock logic and makes it completely deterministic instead of
relying on timed delays.

Bug: 17516245
Change-Id: If2b96845a010f2d05c610970257d898f32ccacb7