fix(boring-sys): localize allocator hooks on Android

[deepso7]

Summary

Avoid weak BoringSSL allocator hooks on Android.

BoringSSL exposes OPENSSL_memory_alloc, OPENSSL_memory_free, and
OPENSSL_memory_get_size as weak ELF symbols. When boring-sys is linked
into an Android shared library alongside other native dependencies, these
symbols can be resolved to incompatible process symbols and cause a crash
during BoringSSL initialization.

This change applies an Android-only patch that uses BoringSSL's existing
local null-function-pointer fallback instead. Other platforms retain the
current weak-symbol behavior.

Verification

• Cross-compiled boring-sys and a downstream UniFFI library for
  aarch64-linux-android.
• Verified the resulting libcrypto.a reports all three allocator hooks as
  LOCAL OBJECT.
• Tested endpoint construction in a React Native/Hermes app on a physical
  arm64 Android device.
• Confirmed BoringSSL initializes successfully without the previous
  SIGSEGV.
• cargo fmt --all -- --check passes.