[droidcamsrc] don't re-activate mode on mode switch if it isn't active
[droidcamsrc] don't re-activate mode on mode switch if it isn't active

If the mode is switched while the camera isn't running, there's no
point in activating it. Also, if we're deactivated in PLAYING->PAUSED
transition, we'll fail to activate because vfsrc pad is flusing.

sailfishos/gst-droid