Actually wait until the display is ready.

When this was rewritten to only wait for display state changes and not
brightness changes, it forgot to take into account whether a change was
already in flight when reporting whether a change was happening. Fix
that so the display appropriately reports when it's ready and when it's
blocked.

Bug: 32559368
Test: Take systrace of device wakeup, see it actually waits until the
      display is actually ready

Change-Id: Ie3bd5c48d109a0fb2628ccc8f67bb52be51fad5c