sysui: refactor for extensibility.

Refactor the test whether touch point is inside
QsContainer into a separate method.
Make mScrollingEnabled protected.

Bug: 28942125
Change-Id: I21a620bbad12c25fedfa5a1139221bd4cb2e95fe
(cherry picked from commit cc10182c291d3e559382ddd181b69c1e89e5ec8e)