Fix crash when a motion event doesn't include locations for all pointers.

When performing a multi-finger gesture, if one finger goes up before the rest of the fingers have finished moving, we need to account for that finger not being present in subsequent motion events.

Bug: 148880038
Bug:136131815
Test: Enable talkback. Enable debugging in TouchExplorer. Perform multi-finger gestures in such a way that one finger goes up just before the other fingers finish moving. Inspect logs.
Test: atest GestureManifoldTest AccessibilityGestureDetectorTest TouchExplorerTest
Change-Id: Ic3158301be657f39232db9f5b3006f87d8d83c4f