FM: Expose method to close FM device file descriptor

The FD is currently not closed when exiting the FM app (android_hardware_fmradio_FmReceiverJNI_closeFdNative
is never called). If there is some other way to do it that I'm missing please let me know.

Change-Id: I32abec63b570190a73157f5bc1f07a5edea691e1