Make BaseJavadocTagClasses descend into nodes

Prior to this change, BaseJavadocTagClasses would stop visiting when
it encountered a class declaration, which meant that inner classes
would never get visited.

Also add a bunch of nested classes to the API whitelist to ensure they
continue to be part of the public API.

Test: external/conscrypt/srcgen/generate_android_src.sh
Test: make -j48 update-api && manually confirm current.txt unchanged
Change-Id: I9a8c14fe3794f81ca4d45e3c9cf6b75c76112633
78 files changed