Add dumpsys to dock observer.

Minor refactoring of dock observer to allow its state to be
inspected and modified via dumpsys for debugging purposes.

eg. View current state.
adb shell dumpsys DockObserver

eg. Simulate being docked.
adb shell dumpsys DockObserver set state 1

eg. Reset back to normal.
adb shell dumpsys DockObserver reset

Change-Id: Ie48db775290ebed9aa4d9d9d5ac5a6fcb6122ac9