[Motion] Fix zero-length motion path container transform bug.

getPosTan, which calculates the translation of the conatienr bounds, no-ops if the startView doesn't move to a new position (when the startView and endView are top aligned and centered horizontally on one another). This was causing motionPathPositions, the int array holding the x and y of the animated bounds to never be initialized, defaulting to 0, and throwing off the bounds calculation.
This CL updates the int array with actual default x and y values so when getPosTan no-ops, the motionPathPosition array contains the correct values.

Resolves #1161

PiperOrigin-RevId: 319247326
(cherry picked from commit 3c9096d)

Author: hunterstich

lib/java/com/google/android/material/transition/MaterialContainerTransform.java

@@ -1139,6 +1139,12 @@ private TransitionDrawable(
       Path motionPath = pathMotion.getPath(startPoint.x, startPoint.y, endPoint.x, endPoint.y);
       motionPathMeasure = new PathMeasure(motionPath, false);
       motionPathLength = motionPathMeasure.getLength();
+      // Fill the motion path with default positions in case a zero-length path is specified which
+      // causes motionPathMeasure.getPosTan to skip filling this int array.
+      // A zero-length path happens when the startBounds are top aligned and horizontally centered
+      // on the endBounds.
+      motionPathPosition[0] = startBounds.centerX();
+      motionPathPosition[1] = startBounds.top;
 
       scrimPaint.setStyle(Paint.Style.FILL);
       scrimPaint.setShader(createColorShader(scrimColor));

lib/java/com/google/android/material/transition/platform/MaterialContainerTransform.java

@@ -1144,6 +1144,12 @@ private TransitionDrawable(
       Path motionPath = pathMotion.getPath(startPoint.x, startPoint.y, endPoint.x, endPoint.y);
       motionPathMeasure = new PathMeasure(motionPath, false);
       motionPathLength = motionPathMeasure.getLength();
+      // Fill the motion path with default positions in case a zero-length path is specified which
+      // causes motionPathMeasure.getPosTan to skip filling this int array.
+      // A zero-length path happens when the startBounds are top aligned and horizontally centered
+      // on the endBounds.
+      motionPathPosition[0] = startBounds.centerX();
+      motionPathPosition[1] = startBounds.top;
 
       scrimPaint.setStyle(Paint.Style.FILL);
       scrimPaint.setShader(createColorShader(scrimColor));