Fix whitespace removal bug

The implementation of whitespace removal in
performHandwritingDeleteGesture had a bug causing an
IndexOutOfBoundsException when the whitespace was at the start or end of
the text.

This also fixes the whitespace removal code in
performHandwritingJoinOrSplitGesture to properly handle supplementary
characters.

Bug: 243983058
Test: atest android.widget.cts.TextViewHandwritingGestureTest
Change-Id: I3cfef91d05d5619668ccfc0d1259b6b1fd96bed9