Android/fix evalue string serialization

[kirklandsign]

Summary

toByteArray() used toString() (returning "EValue@...") instead of
toStr(), and allocated the buffer using char count instead of byte
count. fromByteArray() threw an exception for strings instead of
deserializing them. Fix both directions using a length-prefixed
format, and also fix the misleading "Unknown Tensor dtype" error
message. Add round-trip tests for ASCII, empty, and Unicode strings.

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17609

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❌ 4 New Failures, 1 Unrelated Failure

As of commit 8d18fec with merge base cf08087 ():

NEW FAILURES - The following jobs have failed:

• pull / android / build-llm-demo / linux-job (gh)
  RuntimeError: Command docker exec -t f84e1dc613587326a9b00e62caa1995cd5ee46bc7fc6f68e9ae2cbf849b7f940 /exec failed with exit code 1
• pull / test-mediatek-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 906fb60cba9a8ca73ae4546ea4c51b0fa3558ca5441dc3097866a5978d89f79a /exec failed with exit code 2
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 76d9413c0084792bd2269873646cc811d337808406c820fee3f1f95fde0e1ecd /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7068873436b09f08c97e5147c93e97d2b8c1d524da8986e509caa46b73f9a42c /exec failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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