PhoneWindowManager: add ability to use EdgeGestureService for system gestures.

Update: Default to enabled

Change-Id: Id4e2c0d82f771cd1c20e9196fb6db347494cca69

Conflicts:
	core/java/android/provider/Settings.java
	policy/src/com/android/internal/policy/impl/PhoneWindowManager.java
	services/java/com/android/server/gesture/EdgeGestureTracker.java