Pie controls: Reduce observer footprint of PieController

Based on former commit "Framworks: disable observer and
receiver if PIE is disabled" this commit reduces the observer
footprint of the new pie controller when disabled.

Change-Id: I866ad927c8b99ffa92f0d72bfe20215584649133

Conflicts:

	packages/SystemUI/src/com/android/systemui/statusbar/policy/PieController.java