Android: LlmModule Closeable lifecycle and ModelRunner crash fix (#19012)

## Summary
- **LlmModule**: Implement `Closeable` so modules can be used with
try-with-resources. Add `close()` with explicit
caller contract. Deprecate `resetNative()` in favor of `close()`. Add
`ReentrantLock` to serialize access to
  non-thread-safe native state, matching `Module.java`'s pattern. 
- **ModelRunner**: Add early return after `loadMethod()` failure to
prevent cascading crash in warmup/inference
loops. Fix native memory leak by wrapping benchmark body in try/finally
for `module.destroy()`.
  
### LlmModule lifecycle design
- `ReentrantLock` serializes all operations — prevents accidental
concurrent access (e.g. double-tap, rotation
during generation) from causing native crashes
  - `generate()`/`prefill*()`/`load()`/`resetContext()` acquire the lock
- `close()` uses `tryLock()` — fails fast with `IllegalStateException`
instead of blocking (no ANR risk)
- `stop()` remains a bare native method — lock-free, uses C++ atomic
flag for cross-thread interrupt
- Caller contract: call `stop()`, wait for `generate()` to return, then
`close()`
  ### Tests added                                                 
- `testUseAfterCloseThrows`: `generate()` after `close()` throws
`IllegalStateException`
  - `testCloseIsIdempotent`: double `close()` is safe             
  ## Test plan
- [ ] Existing `LlmModuleInstrumentationTest` tests pass
  - [ ] New lifecycle tests pass                                  
  - [ ] ModelRunner benchmark handles load failures gracefully

  This PR was authored with the help of Claude.

Author: psiddh

extension/android/executorch_android/src/androidTest/java/org/pytorch/executorch/LlmModuleInstrumentationTest.kt

@@ -271,6 +271,22 @@ class LlmModuleInstrumentationTest : LlmCallback {
     }
   }
 
+  // --- Lifecycle tests ---
+
+  @Test
+  fun testUseAfterCloseThrows() {
+    llmModule.close()
+    assertThrows(IllegalStateException::class.java) {
+      llmModule.generate(TEST_PROMPT, SEQ_LEN, this@LlmModuleInstrumentationTest)
+    }
+  }
+
+  @Test
+  fun testCloseIsIdempotent() {
+    llmModule.close()
+    llmModule.close()
+  }
+
   companion object {
     private const val TEST_FILE_NAME = "/stories.pte"
     private const val TOKENIZER_FILE_NAME = "/tokenizer.bin"