fix: use current_thread tokio runtime — avoid pthread_create SEGV on Android

Multi-thread tokio runtime crashes with SEGV_ACCERR in __init_tcb
during pthread_create on Android (static bionic stubs from CRT).
Switch to current_thread runtime which runs network I/O on the
calling thread without spawning additional OS threads.

Also: clean up build.rs — use only libc++_shared.so (dynamic),
remove getauxval_fix.c hack, remove static c++/c++abi linking.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-android/cpp/getauxval_fix.c

@@ -1,21 +0,0 @@
-// Override the broken static getauxval from compiler-rt/CRT.
-// The static version reads from __libc_auxv which may be NULL in shared libs.
-// This version calls the real bionic getauxval via dlsym.
-#ifdef __ANDROID__
-#include <dlfcn.h>
-#include <stdint.h>
-
-typedef unsigned long (*getauxval_fn)(unsigned long);
-
-unsigned long getauxval(unsigned long type) {
-    static getauxval_fn real_getauxval = (getauxval_fn)0;
-    if (!real_getauxval) {
-        real_getauxval = (getauxval_fn)dlsym((void*)-1 /* RTLD_DEFAULT */, "getauxval");
-        if (!real_getauxval) {
-            // Fallback: return 0 (no features detected)
-            return 0;
-        }
-    }
-    return real_getauxval(type);
-}
-#endif