Refactored InteractionSessionId.

Rather than use a local and global id, just use a local, unique id.

Such change not only simplifies the code, but makes it consistent with
TextClassificationSessionId.

Test: mmm -j packages/experimental/FillService && \
      adb install -r ${OUT}/data/app/FillService/FillService.apk && \
      adb shell settings put secure intel_service foo.bar.fill/.AiaiService
Bug: 111276913

Change-Id: I0e610f825aab1d2b32bbafa1bd3d3c7897d889d6