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
Signed-off-by: Neill Kapron <nkapron@google.com>
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:da2fa312c3d53cf96b31c346f56a3ba7a8f99b75)
Merged-In: I565a32594f5eb70efebd460f03757c57daf4b71a
Change-Id: I565a32594f5eb70efebd460f03757c57daf4b71a