Add unit tests covering USB data enable/disable scenarios

This unit test targets UsbService.enableUsbData() and UsbService.enableUsbDataWhileDocked(), covering the following scenarios:

Disabling takes precedence: Disabling requested by requester A overrides a subsequent enabling request from requester B.
Self-cancellation: requester A can cancel their own disabling request by requesting enabling.

Bug: 304579252
Test: atest UsbTests:UsbServiceTest
Change-Id: I52f13c29f87e8bd45459a5a698f96d0ab7a5650f