Android: Add string EValue input support in JNI execute path

[psiddh]

The JNI execute_method handles Tensor, Int, Double, and Bool input types but was missing String support. Java EValue supports String (TYPE_CODE_STRING = 2) and the C++ output path already handles string EValues, but passing a string input would fail with "Unsupported input EValue type code".

String data and ArrayRef are heap-allocated via unique_ptr to ensure pointer stability as the vectors grow, since EValue stores a raw ArrayRef* that must remain valid through execution.

[pytorch-bot]

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❌ 5 New Failures, 3 Unrelated Failures

As of commit f19e1a2 with merge base fe98297 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
  >>> Lint for extension/android/jni/jni_layer.cpp:
• pull / unittest / linux / linux-job (gh)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model
• pull / unittest-editable / linux / linux-job (gh)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph
• pull / unittest-editable / macos / macos-job (gh)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe

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[Copilot]

Pull request overview

This PR adds support for passing Java EValue strings through the Android JNI execute_method input conversion path, bringing input handling in line with the already-supported string output path.

Changes:

• Adds JNI-side handling for JEValue::kTypeCodeString inputs by converting Java String to a native EValue string (ArrayRef<char>*).
• Introduces heap-backed keepalive storage (unique_ptr) for string data and ArrayRef<char> objects to maintain pointer stability during execution.

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