Uses windowContext for the MagnificationGestureHandler

ContextImpl checks the incorrect Context usage in S.
If non-ui context access window service, it would throw an
IllegalAccessException.
And the bug happened because the GestureDectectors in
PanningScalingHandler.class don't have the correct context.
So we have to apply using uiContext for the GestureDectectors
and MagnificationGestureHandler.

Bug: 185154435
Test: none
Change-Id: I942192011526d0e4ce68e6121855bd321c499f56