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)
diff --git a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java
index 45e94f7..62c0fa9 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java
@@ -306,7 +306,7 @@
 
         // Calculate quick setting heights.
         int oldMaxHeight = mQsMaxExpansionHeight;
-        mQsMinExpansionHeight = mKeyguardShowing ? 0 : mQsContainer.getHeader().getHeight();
+        mQsMinExpansionHeight = mKeyguardShowing ? 0 : mQsContainer.getQsMinExpansionHeight();
         mQsMaxExpansionHeight = mQsContainer.getDesiredHeight();
         positionClockAndNotifications();
         if (mQsExpanded && mQsFullyExpanded) {