Fix static analyzer warnings

This fixes warnings like:

runtime/arch/*/context_*.h: warning: Call to virtual function during
construction will not dispatch to derived class

These occur because clang can't tell whether these Context subclasses
themselves have subclasses. Since they're effectively final already, the
simplest fix is to mark them as final.

Bug: 32619234
Test: test-art-host. Rebuilt ART with the analyzer to verify that these
issues are gone.
Change-Id: Idcdd8387e16869564999ca318692dfcc90006b8d