Themes: PhoneWindowManager UI context on demand

Creating the UI context in init causes some elements
like the power menu to not theme correctly. This solution follows
the same pattern as VolumePanel.

Change-Id: I5c12f9a3bc8f5fe8d96ef2775b840926a6c35e28

Conflicts:
	policy/src/com/android/internal/policy/impl/PhoneWindowManager.java