ActiveDisplay: fix concurrency issues

Since we're using the screen state to determine whether to show a notification event
and also turn the screen on it probably should have been a static volatile member in the original
source to prevent threads creating local copies of the screen state in their thread cache when
waking the device with postDelayed. To fix this issue i've added synchronization blocks to handle
all shownotification calls.

Change-Id: I9fbe37103148845ebc3fbd72755fc9d3a97e73ed