Fix mouse key not moving at minimum max speed

Fix the issue that when the mouse keys max speed setting is at
its minimum value, the mouse keys does not moving in diagonal
directions becasue the mMaxMovementStep is smaller than 1.

Bug: 430967163
Test: atest MouseKeysInterceptorTest
Flag: com.android.server.accessibility.enable_mouse_key_enhancement
Change-Id: I5ae95e911c36f566119305e36fe7c9095a476d1e