Suppress redundant power key presses while turning screen on.

Sometimes it can take a long time to turn the screen on.  That's not
good but what's worse is that the user will often get impatient
and press the power button again which effectively cancels the
process and makes the screen not come on at all.  The user may
then conclude that the device ignored the power key press.

This change suppresses power key presses while we're already in the
process of turning the screen on so that at least the screen comes
on eventually, even if it's slower than we would like.

Bug: 17753363
Change-Id: I23083873e35a3dcfa5c20b6f870f53c18e465582