Camera: Avoid invalid member variable access from RequestThread

In case the Camera3Device enters a bad state, the
RequestThread can continue to run after disconnect()
completes. This can potentially cause instabilities
because some of the Camera3Device member variables
will become invalid after disconnect() but are still
accessible in code paths triggered by RequestThread.
Avoid using potentially invalid reference by checking
the respective strong pointer.

Bug: 123293729
Test: Manual using application,
Camera CTS

Change-Id: If3305840db89537593370b7f57bccbb257e49cbd