fix(webrtc-sys): replace __sF with POSIX fdopen for NDK 28+

[fraschm1998]

Problem

webrtc-sys/src/android.cpp references __sF, a libc internal symbol that was removed in Android NDK 28. This causes compilation errors when building with NDK 28 or 29:

src/android.cpp:34:17: error: use of undeclared identifier '__sF'
   34 | FILE *stdout = &__sF[1];
src/android.cpp:37:17: error: use of undeclared identifier '__sF'
   37 | FILE *stderr = &__sF[2];

Fix

Replace the __sF references with POSIX fdopen(STDOUT_FILENO, ...) and fdopen(STDERR_FILENO, ...). This:

• Works across all NDK versions (no internal libc symbols)
• Achieves the same goal: providing stdout/stderr symbols when WebRTC examples need them
• Uses standard POSIX APIs

Tested

Builds successfully with NDK 28.0.12916984 and NDK 29.0.14206865 targeting aarch64-linux-android.

Closes #992

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