Fix repositionChild positioning.

Two seperate issues corrected. First top and left
were swapped as parameters to repositionChild. Second
the recent change to update attributes was incomplete.
Updating the attributes fixes the size and scaling
but its also necessary to update the frame in order
to trigger an update to mShownPosition. Extract and
use a method applyGravityAndUpdateFrame to do so.

Bug: 25791641
Change-Id: Id0b98d587e8acf163121b28eb377c4cf83ebc58b