Test API's for mouse pointer speed

- Add getMousePointerSpeed test API
- Expose InputSettings#DEFAULT_POINTER_SPEED as a test API
- Expose pointer speed settings key as a test API

Test: atest VirtualMouseTest
Bug: 319564269
Change-Id: I7dbef785e21dbd054a65420a2d7aac96ce71b937
Merged-In: Ie14a1522905d3de25d69f99029f75e47fa66aa5f