Acquire wakelock until dream service acknowledges dream state change

Fixes a bug where the system can enter suspend before the dream service
can handle the state change.  This manifests in the DozeService failing
to enable the sensors used to determine if it should pulse the screen
by the time the system goes into full suspend.

This doesn't completely address the asynchronous nature of the
DreamManagerService startDream method.  There's still 2 more asynchronous
calls before the ACTION_DREAMING_STARTED is sent out, but testing shows
this is 'good enough'.

OPO-681
Change-Id: Id6ddc33bc367c1a0182df95374817577006a5b98