sysui: refactor for extensibility

Split button creation code from init in ZenModePanel

Bug: 27101250
Change-Id: Id4bbd505b4e311fdcd2d330f20b73d2338fa9686
(cherry picked from commit 80870e93a49c08f4f4e57d2c0902d2308faa3f7a)
1 file changed