core: touch() buffer when detach()ing

Detachable lets a buffer outlive its original lifetime. The new lifetime
is application-controlled. If the application fails to read/close the
stream, then the leak detector wouldn't make clear what code was
responsible for the buffer's lifetime. With this touch, we'll be able to
see detach() was called and thus know the application needs debugging.

Realized when looking at b/364531464, although I think the issue is
unrelated.

core/src/main/java/io/grpc/internal/ReadableBuffers.java

@@ -415,6 +415,7 @@ public final class ReadableBuffers {
     public InputStream detach() {
       ReadableBuffer detachedBuffer = buffer;
       buffer = buffer.readBytes(0);
+      detachedBuffer.touch();
       return new BufferInputStream(detachedBuffer);
     }