Camera: Avoid invalid request thread access

The following code path:
CameraService::CameraInjectionSession::stopInjection()->
Camera3Device::stopInjection()->
Camera3DeviceInjectionMethods::stopInjection
will try to access 'Camera3Device' 'RequestThread'
without checking whether the instance is valid.
If clients trigger the injection stop just after
Camera3Device disconnect, it is possible to access
an invalid/released request thread.

Flag: EXEMPT bugfix
Bug: 426820406
Test: Camera CTS
Change-Id: I192d9c394760de4b5d29045671adeb2bfccfa00e