SystemUI: don't crash when user disable default apps

If the user has disabled the default clock or calendar application,
System UI will crash when the user clicks the Clock or Calendar
from the notification bar. This patch fixes the issue.

Change-Id: Icdc3679cdd35b5b3a24086e1ada67d0defabc5cb

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/Clock.java