feat: add Android custom text tokenizer

[werran2]

Summary

• add a small pure Kotlin SentencePiece tokenizer for the exported Nano tokenizer.model
• support custom text synthesis in the Android ONNX Runtime example UI and engine API
• keep the existing pre-tokenized demo buttons as smoke-test fallbacks
• document custom text usage and add tokenizer unit tests, including an optional real-model comparison test

Validation

• :app:testDebugUnitTest --rerun-tasks
• MOSS_TOKENIZER_MODEL=/path/to/tokenizer.model :app:testDebugUnitTest --rerun-tasks
• :app:assembleDebug --rerun-tasks
• git diff --check

Note: I verified the Android project builds locally, but I did not run a full on-device model synthesis smoke in this PR.