USB: MtpServer: close MtpFfsHandle before delete

In certain cases, the MtpServer destructor deleting the MtpFfsHandle
causes a crash with SIGABRT in the MtpFfsHandle destructor. Try closing
the handle prior to deleting to perform cleanup.

Test: manual, atest mtp_ffs_handle_test
Bug: 365686062
Flag: EXEMPT bugfix
Change-Id: I565a32594f5eb70efebd460f03757c57daf4b71a
Signed-off-by: Neill Kapron <nkapron@google.com>