Adds estimateFlingEndValue to PhysicsAnimator.

Flings do not have a set end value, and decelerate via friction until coming to a halt. However, it's sometimes useful to have an estimate of where that might be - similar logic is used by flingMustReachMinOrMax.

Test: atest SystemUITests
Change-Id: I271a1587a321b97fbbdad1c00605a17342b6e694