rust/gatt: Suppress rust panics on LE disconnection events received

after rust module unloading

Disconnection events received after the rust module is unloaded
(from shutting down the stack) cause the rust module to raise
a panic because the arbiter is no longer initialized.

This change silences LE disconnection events received in this
context.

Bug: 289280004
Test: atest avatar
Change-Id: Iec122a42feed059f6fa1826b625df10e92ad64bb