Remove SerialService JNI layer

This JNI layer is only used to proxy a basic `open` syscall, which we
can now access directly from `Os`. Simplify the implementation to use
this directly, preserving null return semantics for the
ParcelFileDescriptor when the load fails.

Bug: 375264322
Test: SerialChat + presubmit
Flag: EXEMPT refactor
Change-Id: I35296ea33d04ac1372dc8890fc2c6b58cbe7bb28