Added public API alternatives to reflective calls on ScrollBarDrawable

Added public setter/getter APIs on View to set/get the track/thumb
Drawables for the vertical and horizontal scroll bars to support
configuring scrollbars programmatically in addition to xml
attributes

Change-Id: Ic0b42742e8a795fe616887369dd0e592cde5c2fa
Fixes: 123769505
Test: Added View.java test cases to verify new setter/getters