CallAudioRouteController: fix possible  null pointer dereference in onAvailableRoutesChanged()

If the device was disconnected at the time as onAvailableRoutesChanged()
was called, `deviceToAdd` may be NULL.

Bug: b/368222213
Test: Manual
Change-Id: I30bec44956c8f0790114dbc9672d598c2d1291eb
1 file changed