Make buildDirectionalMotionSpec match the style of `motionSpec` builder

- Move to the builder sub-package
- create spatial/effects specific variants on MotionBuilderContext
- remove the `build` from the name

Bug: 406734758
Test: Existing unit tests
Flag: com.android.systemui.scene_container
Change-Id: I9fbb905f805612af474885460906cff61b42d4ed