Minor changes on LocusId APIs.

- Uses a String instead of URI for the main id.
- Uses FLAG_IS_PREFIX instead of isRecursive for UserDataRemovalRequest.

Test: m update-api
Test: atest CtsContentCaptureServiceTestCases CtsContentTestCases:LocusIdTest \
            FrameworksCoreTests:android.view.contentcapture.ContentCaptureEventTest
Bug: 126945732

Change-Id: I34ae42678aa4021ea53ed66279cad4049ac2dcb3