ModPhone: use partial wakelock while periodic vibrations are active
When screen is turned off during phone call, some devices seem to fall
into deep sleep resulting in Handler not being able to process
VIBRATE_45_SEC messages.
Partial wakelock helps Handler to keep processing these messages on
time.
Wakelock is acquired only for a necessary duration which is extended
everytime VIBRATE_45_SEC messages is scheduled. This assures wakelock
will go away by itself in case of some unexpected situation.