Ensure correct width and height are used during popup positioning

Passes the width and height into findDropDownPosition() rather than
relying on global state. Ensures that an update is forced if any
aspects of the LayoutParams are changed during drop down position
computetion. Cleans up method and argument naming.

Bug: 27819843
Change-Id: Id85e2a0e81e0ea6a754dadf7c1d1c2493a5979b0