Safeguard DreamManager in PhoneWindowManager

Currently calls to the DreamManagerInternal inside PhoneWindowManager
rely on an instance being properly cached, which has caused
NullPointerException's in some rare cases. This change ensures that all
references to it within the class go through getDreamManagerInternal()
which fetches the latest service should the local variable be null.

Fix: 401205442
Test: verified that power button press on device after a reboot no
longer crashes
Flag: EXEMPT small bug fix

Change-Id: Ieee5af390a122f90bf7b1bd641d0fff8a8bff380