fix(gestures): Clear VelocityTracker on ACTION_DOWN to prevent stale velocity data

Matches GestureDetector behavior: if consecutive ACTION_DOWN events
arrive without an intervening ACTION_UP/ACTION_CANCEL, stale motion
data could bleed into fling detection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: romtsn

sentry-android-core/src/main/java/io/sentry/android/core/internal/gestures/SentryGestureDetector.java

@@ -50,6 +50,10 @@ boolean onTouchEvent(final @NotNull MotionEvent event) {
     if (velocityTracker == null) {
       velocityTracker = VelocityTracker.obtain();
     }
+
+    if (action == MotionEvent.ACTION_DOWN) {
+      velocityTracker.clear();
+    }
     velocityTracker.addMovement(event);
 
     switch (action) {