sysui: refactor for extensibility.

1. Refactored container height calculation out of updateBottom of QSContainer;

2. Added a method in QSContainer to return min expansion height.

Bug: 27929808
Change-Id: I5a6d04ffb225bccfc56fa5f4ab98d330ed763a8c
(cherry picked from commit 9e874950195d6b76a64e28c236e8d109d551281f)