Add a tab to grab to CropView handles.

Not codifying the handle size in an attribute since the
mock is expected to be replaced so that param may not make sense soon.

Also call getWidth/getHeight where we were calling
getMeasuredWidth/Height before.

Bug: 180967468
Test: Visual verification of half-circle tabs drawn on crop handles

Change-Id: I4716fa2d19c40ce1f825c71f3a135544532bc927