SL-16024: Return shared_ptr from LLInstanceTracker::getInstance().

It feels wrong to return a dumb LLInstanceTracker subclass* from getInstance()
when we use std::shared_ptr and std::weak_ptr internally. But tweak consumers
to use 'auto' or LLInstanceTracker::ptr_t in case we later revisit this
decision.

We did add a couple get() calls where it's important to obtain a dumb pointer.