sysui: refactor for extension

Refactored ZenModePanel:
1. Button creation is split from onFinishInflate.
2. Added a method to query current Zen condition
3. Made mZenButtonsCallback protected.

Change-Id: I959fa2f7770ba1888af01eababe7c4512981332d
(cherry picked from commit 79df200108c5240c0d2a041d7d841f8d9d143ec0)