PhoneWindowPolicy: Fix NPE race condition by excessive screen on delay

When screen on has an excessive delay mTopFullscreenOpaqueWindowState could
not be initialized. This derives in a NPE from a Power key press. If
mTopFullscreenOpaqueWindowState = null then PREVENT_POWER_KEY can be safelly
ignored

Change-Id: I04ce5d88f6f9abf02b2bbcdce4a2acad2f6451fa
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>