Fix issue with lifecycle of Activities launched from Lockscreen

ActivityManager still thinks the device is going to sleep, so
any Activity started while the lockscreen is showing is immediately
stopped. When Keyguard calls ActivityManager.keyguardWaitingForActivityDraw(),
then we must make sure that the ActivityManager thinks we are not sleeping.

Bug:17459745
Change-Id: I5c45c88e3a72e1f08e1b32101d0d0b4675f7a5ff