Change hover/press animation method of caption handle

Changes the value that is animated from |scaleX| to |paddingLeft| and
|paddingRight| in order to preserve semicircular shape of handle endings
at all times.

Fix: 343720907
Flag: com.android.window.flags.enable_desktop_windowing_mode
Test: manual
Change-Id: Id52c17a120c6b22a7cc8cc8243c9d98f802e566f