Remove non-trivial constructors/destructors from SystemProperties

With the goal of disallowing exit time destructors, SystemProperties's
non-trivial destructor needs to be removed.  This means replacing the
union hack with yet another hack as we don't want to allocate anything
despite relying on some polymorphism.

Bug: 73485611
Test: boot bullhead
Change-Id: I64223714c9b26c9724bfb8f3e2b0168e47b56bc8