Simplify calculateEndTarget logic.

This method has grown quite large and has a lot of if-else nesting. We
can address these readability issues by defining some helper methods for
specific cases (e.g. flingY and non-fling).

Bug: 222117127
Test: Manual
Change-Id: Ia6012dbcee9a842b4b5bd2ffdb9aea978cfdfc26