Allow traversal over the trie structure

Previously, there was no way to traverse the trie structure and no way
to identify specific nodes in the trie. That made it impossible to
analyze the trie structure resulting from loading a set of flags. This
change adds type and selector properties to nodes as well as access to
the child nodes of a node to allow for the structure to be analyzed.

Bug: 202154151
Test: m out/soong/hiddenapi/hiddenapi-flags.csv
      atest --host signature_trie_test verify_overlaps_test
      pyformat -s 4 --force_quote_type double -i scripts/hiddenapi/signature_trie*
      /usr/bin/pylint --rcfile $ANDROID_BUILD_TOP/tools/repohooks/tools/pylintrc scripts/hiddenapi/signature_trie*
Change-Id: Ia4714dbf59f6fd143aa3bf3ad1a59cd073d2175b